# 3.2. Solving Quadratic Equations. Introduction. Prerequisites. Learning Outcomes

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1 Solving Quadratic Equations 3.2 Introduction A quadratic equation is one which can be written in the form ax 2 + bx + c = 0 where a, b and c are numbers, a 0, and x is the unknown whose value(s) we wish to find. In this Section we describe several was in which quadratic equations can be solved. Prerequisites Before starting this Section ou should... Learning Outcomes On completion ou should be able to... be able to solve linear equations recognise a quadratic equation solve a quadratic equation b factorisation solve a quadratic equation using the standard formula solve a quadratic equation b completing the square interpret the solution of a quadratic equation graphicall HELM (2008): Section 3.2: Solving Quadratic Equations 13

3 Exercises 1. Verif that x = 2 and x = 3 are both solutions of x 2 5x + 6 = Verif that x = 2 and x = 3 are both solutions of x 2 + 5x + 6 = b factorisation It ma be possible to solve a quadratic equation b factorisation using the method described for factorising quadratic expressions in 1.5, although ou should be aware that not all quadratic equations can be easil factorised. Example 10 Solve the equation x 2 + 5x = 0. Factorising and equating each factor to zero we find x 2 + 5x = 0 is equivalent to x(x + 5) = 0 so that x = 0 and x = 5 are the two solutions. Example 11 Solve the quadratic equation x 2 + x 6 = 0. Factorising the left hand side we find x 2 + x 6 = (x + 3)(x 2) so that x 2 + x 6 = 0 is equivalent to (x + 3)(x 2) = 0 When the product of two quantities equals zero, at least one of the two must equal zero. In this case either (x + 3) is zero or (x 2) is zero. It follows that x + 3 = 0, giving x = 3 or x 2 = 0, giving x = 2 Here there are two solutions, x = 3 and x = 2. These solutions can be checked quite easil b substitution back into the given equation. HELM (2008): Section 3.2: Solving Quadratic Equations 15

4 Example 12 Solve the quadratic equation 2x 2 7x 4 = 0 b factorising the left-hand side. Factorising the left hand side: 2x 2 7x 4 = (2x+1)(x 4) so 2x 2 7x 4 = 0 is equivalent to (2x+ 1)(x 4) = 0. In this case either (2x + 1) is zero or (x 4) is zero. It follows that 2x + 1 = 0, giving x = 1 2 or x 4 = 0, giving x = 4 There are two solutions, x = 1 2 and x = 4. Example 13 Solve the equation 4x x + 9 = 0. Factorising we find 4x x + 9 = (2x + 3)(2x + 3) = (2x + 3) 2 This time the factor (2x + 3) occurs twice. The original equation 4x x + 9 = 0 becomes (2x + 3) 2 = 0 so that 2x + 3 = 0 and we obtain the solution x = 3. Because the factor 2x + 3 appears twice in the equation 2 (2x + 3) 2 = 0 we sa that this root is a repeated solution or double root. Task Solve the quadratic equation 7x 2 20x 3 = 0. First factorise the left-hand side: Your solution 7x 2 20x 3 = Answer (7x + 1)(x 3) Equate each factor is then equated to zero to obtain the two solutions: Your solution 1: x = 2: x = Answer 1 and HELM (2008): Workbook 3: Equations, Inequalities & Partial Fractions

5 Exercises Solve the following equations b factorisation: 1. x 2 3x + 2 = 0 2. x 2 x 2 = 0 3. x 2 + x 2 = 0 4. x 2 + 3x + 2 = 0 5. x 2 + 8x + 7 = 0 6. x 2 7x + 12 = 0 7. x 2 x 20 = x 2 4 = 0 9. x 2 + 2x 1 = x 2 + 6x + 3 = x x = x 2 + 2x = 0 Answers The factors are found to be: 1. 1, , , , , , , , twice twice , , 1 3. Completing the square The technique known as completing the square can be used to solve quadratic equations although it is applicable in man other circumstances too so it is well worth studing. Example 14 (a) Show that (x + 3) 2 = x 2 + 6x + 9 (b) Hence show that x 2 + 6x can be written as (x + 3) 2 9. (a) Removing the brackets we find (x + 3) 2 = (x + 3)(x + 3) = x 2 + 3x + 3x + 9 = x 2 + 6x + 9 (b) B subtracting 9 from both sides of the previous equation it follows that (x + 3) 2 9 = x 2 + 6x HELM (2008): Section 3.2: Solving Quadratic Equations 17

6 Example 15 (a) Show that (x 4) 2 = x 2 8x + 16 (b) Hence show that x 2 8x can be written as (x 4) (a) Removing the brackets we find (x 4) 2 = (x 4)(x 4) = x 2 4x 4x + 16 = x 2 8x + 16 (b) Subtracting 16 from both sides we can write (x 4) 2 16 = x 2 8x We shall now generalise the results of Examples 14 and 15. Noting that (x + k) 2 = x 2 + 2kx + k 2 we can write x 2 + 2kx = (x + k) 2 k 2 Note that the constant term in the brackets on the right-hand side is alwas half the coefficient of x on the left. This process is called completing the square. Ke Point 4 Completing the Square The expression x 2 + 2kx is equivalent to (x + k) 2 k 2 Example 16 Complete the square for the expression x x. Comparing x x with the general form x 2 + 2kx we see that k = 8. Hence x x = (x + 8) = (x + 8) 2 64 Note that the constant term in the brackets on the right, that is 8, is half the coefficient of x on the left, which is HELM (2008): Workbook 3: Equations, Inequalities & Partial Fractions

7 Example 17 Complete the square for the expression 5x 2 + 4x. Consider 5x 2 + 4x. First of all the coefficient 5 is removed outside a bracket as follows 5x 2 + 4x = 5(x x) We can now complete the square for the quadratic expression in the brackets: x x = (x + 2 ( ) )2 = (x ) Finall, multipling both sides b 5 we find ( 5x 2 + 4x = 5 (x )2 4 ) 25 Completing the square can be used to solve quadratic equations as shown in the following Examples. Example 18 Solve the equation x 2 + 6x + 2 = 0 b completing the square. First of all just consider x 2 + 6x, and note that we can write this as x 2 + 6x = (x + 3) 2 9 Then the quadratic equation can be written as x 2 + 6x + 2 = (x + 3) = 0 that is (x + 3) 2 = 7 Taking the square root of both sides gives x + 3 = ± 7 so x = 3 ± 7 The two solutions are x = = and x = 3 7 = , to 4 d.p. HELM (2008): Section 3.2: Solving Quadratic Equations 19

8 Example 19 Solve the equation x 2 8x + 5 = 0 First consider x 2 8x which we can write as x 2 8x = (x 4) 2 16 so that the equation becomes x 2 8x + 5 = (x 4) = 0 i.e. (x 4) 2 = 11 x 4 = ± 11 x = 4 ± 11 So x = or x = (to 4 d.p.) Task Solve the equation x 2 4x + 1 = 0 b completing the square. First examine the two left-most terms in the equation: x 2 4x. Complete the square for these terms: Your solution x 2 4x = Answer (x 2) 2 4 Use the above result to rewrite the equation x 2 4x + 1 = 0 in the form (x?) 2 +? = 0: Your solution x 2 4x + 1 = Answer (x 2) = (x 2) 2 3 = 0 From this now obtain the roots: Your solution Answer (x 2) 2 = 3, so x 2 = ± 3. Therefore x = 2 ± 3 so x = or to 4 d.p. 20 HELM (2008): Workbook 3: Equations, Inequalities & Partial Fractions

9 Exercises 1. Solve the following quadratic equations b completing the square. (a) x 2 3x = 0 (b) x 2 + 9x = 0. (c) 2x 2 5x + 2 = 0 (d) 6x 2 x 1 = 0 (e) 5x 2 + 6x 1 = 0 (f) x 2 + 4x 3 = 0 2. A chemical manufacturer found that the sales figures for a certain chemical X 2 O depended on its selling price. At present, the compan can sell all of its weekl production of 300 t at a price of 600 / t. The compan s market research department advised that the amount sold would decrease b onl 1 t per week for ever 2 / t increase in the price of X 2 O. If the total production costs are made up of a fixed cost of per week, plus 400 per t of product, show that the weekl profit is given b P = x x where x is the new price per t of X 2 O. Complete the square for the above expression and hence find Answers (a) the price which maximises the weekl profit on sales of X 2 O (b) the maximum weekl profit (c) the weekl production rate 1. (a) 0, 3 (b) 0, 9 (c) 2, 1 (d) 1, 1 (e) 1, 1 (f) 1, (a) 800 / t, (b) /wk, (c) 200 t / wk 4. b formula When it is difficult to factorise a quadratic equation, it ma be possible to solve it using a formula which is used to calculate the roots. The formula is obtained b completing the square in the general quadratic ax 2 + bx + c. We proceed b removing the coefficient of a: ax 2 + bx + c = a{x 2 + b a x + c a } = a{(x + b 2a )2 + c a b2 4a 2 } Thus the solution of ax 2 + bx + c = 0 is the same as the solution to (x + b 2a )2 + c a b2 4a 2 = 0 HELM (2008): Section 3.2: Solving Quadratic Equations 21

10 So, solving: (x + b 2a )2 = c a + b2 which leads to x = b 4a 2 2a ± c a + b2 4a 2 Simplifing this expression further we obtain the important result: Ke Point 5 Quadratic Formula If ax 2 + bx + c = 0, a 0 then the two solutions (roots) are x = b b 2 4ac 2a and x = b + b 2 4ac 2a To appl the formula to a specific quadratic equation it is necessar to identif carefull the values of a, b and c, paing particular attention to the signs of these numbers. Substitution of these values into the formula then gives the desired solutions. Note that if the quantit b 2 4ac (called the discriminant) is a positive number we can take its square root and the formula will produce two values known as distinct real roots. If b 2 4ac = 0 there will be one value onl known as a repeated root or double root. The value of this root is x = b 2a. Finall if b2 4ac is negative we sa the equation possesses complex roots. These require special treatment and are described in 10. Ke Point 6 When finding roots of the quadratic equation ax 2 + bx + c = 0 first calculate the discrinimant b 2 4ac If b 2 4ac > 0 the quadratic has two real distinct roots If b 2 4ac = 0 the quadratic has two real and equal roots If b 2 4ac < 0 the quadratic has no real roots: there are two complex roots 22 HELM (2008): Workbook 3: Equations, Inequalities & Partial Fractions

11 Example 20 Compare each given equation with the standard form ax 2 +bx+c = 0 and identif a, b and c. Calculate b 2 4ac in each case and use this information to state the nature of the roots. (a) 3x 2 + 2x 7 = 0 (b) 3x 2 + 2x + 7 = 0 (c) 3x 2 2x + 7 = 0 (d) x 2 + x + 2 = 0 (e) x 2 + 3x 1 2 = 0 (f) 5x2 3 = 0 (g) x 2 2x + 1 = 0 (h) 2p 2 4p = 0 (i) p 2 + 4p 4 = 0 (a) a = 3, b = 2, c = 7. So b 2 4ac = (2) 2 4(3)( 7) = 88. The roots are real and distinct. (b) a = 3, b = 2,c = 7. So b 2 4ac = (2) 2 4(3)(7) = 80. The roots are complex. (c) a = 3, b = 2, c = 7. So b 2 4ac = ( 2) 2 4(3)(7) = 80. The roots are complex. (d) a = 1, b = 1, c = 2. So b 2 4ac = 1 2 4(1)(2) = 7. The roots are complex. (e) a = 1, b = 3, c = 1 2. So b2 4ac = 3 2 4( 1)( 1 2 ) = 7. The roots are real and distinct. (f) a = 5, b = 0, c = 3. So b 2 4ac = 0 4(5)( 3) = 60. The roots are real and distinct. (g) a = 1, b = 2, c = 1. So b 2 4ac = ( 2) 2 4(1)(1) = 0. The roots are real and equal. (h) a = 2, b = 4, c = 0. So b 2 4ac = ( 4) 2 4(2)(0) = 16 The roots are real and distinct. (i) a = 1, b = 4, c = 4. So b 2 4ac = ( 4) 2 4( 1)( 4) = 0 The roots are real and equal. HELM (2008): Section 3.2: Solving Quadratic Equations 23

12 Example 21 Solve the quadratic equation 2x 2 + 3x 6 = 0 using the formula. We compare the given equation with the standard form ax 2 + bx + c = 0 in order to identif a, b and c. We see that here a = 2, b = 3 and c = 6. Note particularl the sign of c. Substituting these values into the formula we find x = b ± b 2 4ac 2a = 3 ± 3 2 4(2)( 6) (2)(2) = 3 ± = 3 ± Hence, to 4 d.p., the two roots are x = , if the positive sign is taken and x = if the negative sign is taken. However, it is often sufficient to leave the solution in the so-called surd form x = 3 ± 57, which is exact. 4 Task Solve the equation 3x 2 x 6 = 0 using the quadratic formula. First identif a, b and c: Your solution Answer a = b = c = a = 3, b = 1, c = 6 Substitute these values into the formula and simplif: Your solution x = b ± b 2 4ac 2a Answer ( 1) ± ( 1) 2 (4)(3)( 6) (2)(3) Finall, calculate the values of x to 4 d.p.: Your solution Answer so x = = 1 ± 73 6 x = or x = , HELM (2008): Workbook 3: Equations, Inequalities & Partial Fractions

13 Engineering Example 1 Undersea cable fault location Introduction The voltage (V ), current (I) and resistance (R) in an electrical circuit are related b Ohm s law i.e. V = IR. If there are two resistances (R 1 and R 2 ) in an electrical circuit, the ma be in series, in which case the total resistance (R) is given b R = R 1 + R 2. Or the ma be in parallel in which case the total resistance is given b 1 R = 1 R R 2 In 1871 the telephone cable between England (A) and Denmark (B) developed a fault, due to a short circuit under the sea (see Figure 2). Oliver Heaviside, an electrical engineer, came up with a ver simple method to find the location of the fault. He assumed that the cable had a uniform resistance per unit length. Heaviside performed two tests: (1) connecting a batter (voltage E) at A, with the circuit open at B, he measured the resulting current I 1, (2) connecting the same batter at A, with the cable earthed at B, he measured the current I 2. A x r x B England Denmark short-circuit Figure 2: Schematic of the undersea cable In the first measurement the resistances up to the cable fault and between the fault and the short circuit are in series and in the second experiment the resistances beond the fault and between the fault and the short circuit are in parallel. Problem in words Use the information from the measurements to deduce the location of the fault. Mathematical statement of problem (a) Denote the resistances of the various branches b the smbols shown in Figure 2. (b) Use Ohm s law to write down expressions that appl to each of the two measurements. (c) Eliminate from these expressions to obtain an expression for x. HELM (2008): Section 3.2: Solving Quadratic Equations 25

14 Mathematical analsis (a) In the first experiment the total circuit resistance is x +. In the second experiment, the total circuit resistance is given b: ( 1 x + r x + 1 ) 1 So application of Ohm s law to each experimental situation gives: E = I 1 (x + ) ( 1 E = I 2 (x + r x + 1 ) 1 ) Rearrange Equation (1) to give E I 1 x = Substitute for in Equation (2), divide both sides b I 2 and introduce E I 1 = r 1 and E I 2 = r 2 : r 2 = (x + ( 1 r x + 1 ) 1 ) Use a common denominator for the fractions on the right-hand side: ( ) (r x)(r1 x) r 2 = (x + ) = x(r 1 + r 2x) + (r x)(r 1 x) r 1 x + r x (r 1 + r 2x) Multipl through b (r 1 + r 2x): r 2 (r 1 + r 2x) = x(r 1 + r 2x) + (r x)(r 1 x) Rearrange as a quadratic for x: x 2 2r 2 x rr 1 + r 2 r 1 + rr 2 = 0 Use the standard formula for solving quadratic equations with a = 1, b = 2r 2 and c = rr 1 + r 2 r 1 + rr 2 : x = 2r 2 ± 4r2 2 4( rr 1 + r 2 r 1 + rr 2 ) = r 2 ± (r r 2 )(r 1 r 2 ) 2 Onl positive solutions would be of interest. (1) (2) 26 HELM (2008): Workbook 3: Equations, Inequalities & Partial Fractions

15 Engineering Example 2 Estimating the mass of a pipe Introduction Sometimes engineers have to estimate component weights from dimensions and material properties. On some occasions, engineers prefer use of approximate formulae to exact ones as long as the are sufficientl accurate for the purpose. This Example introduces both of these aspects. Problem in words (a) Find the mass of a given length of pipe in terms of its inner and outer diameters and the densit of the pipe material. (b) Find the wall thickness of the pipe if the inner diameter is 0.15 m, the densit is 7900 kg m 3 and the mass per unit length of pipe is 40 kg m 1. (c) Find an approximate method for calculating the mass of a given length of a thin-walled pipe and calculate the maximum ratio of inner and outer diameters that give an error of less than 10% when using the approximate method. Mathematical statement of problem (a) Denote the length of the pipe b L m and inside and outside diameters b d i m and d o m, respectivel and the densit b ρ kg m 3. Assume that the pipe is clindrical so its cross section corresponds to the gap between concentric circles (this is called an annulus or annular region - see 2.6). Calculate the difference in cross sectional areas b using the formula for the area of a circle (A = πr 2 where r is the radius) and multipl b the densit and length to obtain mass (m). (b) Rearrange the equation in terms of wall thickness (d m) and inner diameter. Substitute the given values to determine the wall thickness. (c) Approximate the resulting expression for small values of (d o d i ). Calculate the percentage difference in predictions between the original and approximate formulae for various numerical values of d i /d o. Mathematical analsis (a) The cross section of a clindrical pipe is a circular annulus. The area of a circle is given b πr 2 = π 4 d2, since r = d/2 if d is the diameter. So the area of the outer circle is π 4 d2 o and that of the inner circle is π 4 d2 i. This means that the mass m kg of length L m of the pipe is given b m = π 4 (d2 0 d 2 i )Lρ HELM (2008): Section 3.2: Solving Quadratic Equations 27

16 (b) Denote the pipe wall thickness b δ so d o = d i Use (d 2 o d 2 i ) = (d o d i )(d o + d i ) = 2δ(2d i + 2δ). So m = πδ(d i + δ)lρ Given that m/l = 40, d i = 0.15 and r = 7900, 40 = pd( d)7900. Rearrange this equation as a quadratic in δ, δ δ 4π/790 = 0 Solve this quadratic using the standard formula with a = 1, b = 0.15 and c = 4π/790. Retain onl the positive solution to give δ = 0.072, i.e. the pipe wall thickness is 72 mm. (c) If δ is small then (d o d i ) is small and d i + δ d i. So the expression for m in terms of δ ma be written m πδd i Lρ The graph in Figure 3 shows that the percentage error from using the approximate formula for the mass of the pipe exceeds 10% onl if the inner diameter is less than 82% of the outer diameter. The percentage error from using the approximate formula can be calculated from (exact result approximate result)/(exact result) 100% for various values of the ratio of inner to outer diameters. In the graph the error is plotted for diameter ratios between 0.75 and % error 10 5 Comment Inner diameter / Outer diameter Figure 3 The graph shows also that the error is 1% or less for diameter ratios > HELM (2008): Workbook 3: Equations, Inequalities & Partial Fractions

17 Exercises Solve the following quadratic equations b using the formula. Give answers exactl (where possible) or to 4 d.p.: 1. x 2 + 8x + 1 = 0 2. x 2 + 7x 2 = 0 3. x 2 + 6x 2 = 0 4. x 2 + 3x + 1 = x 2 3x + 1 = x 2 + 5x 3 = 0 Answers , , , , , , 3 5. Geometrical representation of quadratics We can plot a graph of the function = ax 2 + bx + c (given the values of a, b and c). If the graph crosses the horizontal axis it will do so when = 0, and so the x coordinates at such points are solutions of ax 2 + bx + c = 0. Depending on the sign of a and of the nature of the solutions there are essentiall six different tpes of graph that can occur. These are displaed in Figure 4. real, distinct roots real, equal roots complex roots a > 0 x x x a < 0 x x x Figure 4: The possible graphs of a quadratic = ax 2 + bx + c Sometimes a graph of the quadratic is used to locate the solutions; however, this approach is generall inaccurate. This is illustrated in the following example. HELM (2008): Section 3.2: Solving Quadratic Equations 29

18 Example 22 Solve the equation x 2 4x + 1 = 0 b plotting a graph of the function: = x 2 4x + 1 B constructing a table of function values we can plot the graph as shown in Figure 5. x C D x Figure 5: The graph of = x 2 4x + 1 cuts the x axis at C and D The solutions of the equation x 2 4x + 1 = 0 are found b looking for points where the graph crosses the horizontal axis. The two points are approximatel x = 0.3 and x = 3.7 marked C and D on the Figure. Exercises 1. Solve the following quadratic equations giving exact numeric solutions. Use whichever method ou prefer (a) x 2 9 = 0 (b) s 2 25 = 0 (c) 3x 2 12 = 0 (d) x 2 5x + 6 = 0 (e) 6s 2 + s 15 = 0 (f) p 2 + 7p = 0 2. Solve the equation 2x 2 3x 7 = 0 giving solutions rounded to 4 d.p. 3. Solve the equation 2t 2 + 3t 4 giving the solutions in surd form. Answers 1(a) x = 3, 3, (b) s = 5, 5, (c) x = 2, 2, (d) x = 3, 2, (e) s = 3/2, 5/3, (f) p = 0, , ± HELM (2008): Workbook 3: Equations, Inequalities & Partial Fractions

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