# Block 7: Applications of Differentiation Contents

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1 Block 7: Applications of Differentiation Contents 7.1 Optimisation Velocity and Acceleration Differential Equations First Order Differential Equations Second Order Differential Equations Answers to Exercises... 12

2 2 7.1 Optimisation Stationary Points Reminder In F11EP1 you were taught how to locate the stationary points of a function f(x) and determine their nature (whether a stationary point was a maximum or a minimum). Here is a reminder: If f (c) = 0 then c is a stationary point. There are two methods for finding the nature of a stationary point at x = c. The first method compares the sign of f (x) for x<c and x>c (with x close to c ). For a maximum, f (x) is positive for x<c and negative for x>c. For a minimum, f (x) is negative for x<c and positive for x>c. In this module, we focus on the method called the second derivative test. Since f (x) measures the rate at which f (x) increases, if f (c) < 0 then f (x) is decreasing at c.if f (c) > 0 then f (x) is increasing at c. Hence if f (c) = 0 and f (c) < 0 then x = c is a local maximum. if f (c) = 0 and f (c) > 0 then x = c is a local minimum. Optimisation Problems In many real world problems we want to make something as large or as small as possible by changing some of the parameters in the problem ( optimisation ). We can use our knowledge for finding and classifying stationary points to do this. Example 7.1 A variable rectangle has a constant perimeter of 16. Find the lengths of the sides when the area is a maximum. Solution There are many rectangles with a perimeter 16, for example sides 7 and 1 (area 7 ) or 5 and 3 (area 15 ). To do problems like this we have to express the given information about the area as a function of one variable. If the sides are x and y then the area is A = xy. Since 2x +2y= 16 we have that A(x) =x(8 x) =8x x 2. At stationary points A (x) = 0. Then 8 2x = 0 and x = 4 is the stationary point. Since A (x) = 2, A (4) < 0 so x = 4 is a maximum. Hence the area is a maximum when it is a square of side 4. Example 7.2 An open rectangular tank with a square base is to be constructed so that it has a volume of 500 cubic metres. Find the dimensions of the tank if the total surface area is a minimum. Solution Let h be the height and x the length of the side of the base. The surface area S is the area of the four walls and base, that is S = x 2 +4xh. To proceed we need to use the information about the volume to get S as a function of one variable.

3 3 Since the volume is 500 we have x 2 h = 500 so and h = 500 x 2 ( ) 500 S(x) =x 2 +4x x 2 = x x Then S (x) =2x 2000 =0 x 2 if x = 10. Hence x = 10 is the stationary point. Since S (x) = x 3,S (10) > 0 and x = 10 is a minimum. Using x 2 h = 500, h = 5 and the required dimensions are 10 by 10 by 5. Exercises A variable rectangle has a constant area of 49. Find the lengths of the sides when the perimeter is a minimum. 2. A rectangular block has a square base. Its total surface area is 150. (a) If the base length is x, show that the volume V of the block is V = 75x x3 2 (b) Find the dimensions of the block if the volume is a maximum. 3. An open box is to be made with a square base. The volume of the box is 32 cm 3. Find the dimensions of the box if the surface area of the box is a minimum. 4. A rectangular box-shaped house is to have a square floor. Four times as much heat per square metre is lost through the roof as through the walls: no heat is lost through the floor. The house has to enclose 2000 cubic metres. Find the dimensions of the house so as to minimise heat loss. 5. A rectangular field is to be fenced off along a road. The fence along the road costs 3 pounds per metre while on the other sides it costs 2 pounds per metre. Calculate the maximum area that can be fenced off for 400 pounds.

4 4 7.2 Velocity and Acceleration Suppose that a car is travelling in a straight line and that after time t it is at a distance s(t) from its starting point. If the velocity v(t) is constant then the graph of s(t) against t is a straight line and the acceleration of the car is zero. If the velocity v(t) is not constant then the graph of s(t) against t will not be a straight line. The average velocity of the car in the time interval t to t + h is given by change in distance change in time = s(t + h) s(t) h The velocity of the car at the exact time t can be approximated by making h small so that the average velocity is calculated over a small time interval. Taking the limit, the velocity at time t is given by v(t) =s (t). Similarly, the acceleration at time t is given by a(t) =v (t). Example 7.3 A particle moves along a straight line such that its distance s(t) metres from a fixed point P after t seconds is given by s(t) =t 3 3t 2 +9t. (a) Find the distance of the particle from P after 2 seconds. (b) Find the velocity and acceleration after t seconds. (c) Find the velocity when the acceleration of the particle is zero. Solution (a) s(t) is the distance after t seconds so s(2) = 2 3 3(2 2 ) + 9(2) = 14 metres is the distance from P after 2 second. (b) v(t) =s (t)=3t 2 6t+ 9 and a(t) =v (t)=6t 6. (c) If the acceleration of the particle is zero then a(t) = 0. 6t 6=0 so t= 1. The velocity at this time is v(1)= = 6 m/s. Example 7.4 A missile is fired vertically upwards and after t seconds its height s(t) in metres above the ground is given by s(t) = t 5t 2. Find (a) the initial velocity of the missile (b) the maximum height reached by the missile. (c) the velocity of the missile when it hits the ground. Solution (a) v(t) =s (t) = t So initial velocity (when t =0)isv(0) = 100ms 1 (b) Maximum height is when v (t) = 0. This is when t = 0 which means t =10s. Height when t =10iss(10) = = 525m. (c) Missile hits the ground when s(t) = 0. Using the quadratic formula this is when t = 100 ± = 10 ± = 0.247s or s 10

5 5 The negative time here is not of interest as it is before the missile has been fired. We require v(20.247) = ms 1. The missile has a speed of ms 1 and is travelling vertically down (hence the negative velocity). Example 7.5 The height, h, of a mass on a spring (under the action of gravity, but without air resistance or friction) is h = A cos(ωt). Here A is the amplitude and 2π/ω is the period of oscillation. Find (a) the velocity and acceleration of the mass. (b) the maximum height of the mass. (c) the maximum velocity of the mass. Solution (a) Velocity, v(t) =h (t)= Aω sin(ωt). Acceleration, a(t) = Aω 2 cos(ωt). (b) Maximum height when v(t) = 0 which is when Aω sin(ωt) = 0. This is when t =0,2π/ω, 4π/ω,. Using t = 0 the maximum height is h(0) = A cos(0) = A. (c) Maximum velocity is when a(t) = 0 which is when Aω 2 cos(ωt) = 0. This is when t =3π/2ω, 7π/2ω,. Using t =3π/2ω the maximum velocity is Aω sin(3π/2ω) =Aω. Exercises A particle moves along a straight line such that its distance s(t) from a fixed point P after t seconds is given by s(t) =t 3 6t Find (a) its distance from P after 1 second, (b) its acceleration at the time t>0 when its velocity is zero. 2. A ball is thrown vertically upwards and after t seconds its height s(t) above the ground is given by s(t) =8+10t 5t 2. Find (a) the height from which the ball is thrown (b) the initial velocity of the ball (c) the maximum height reached by the ball. 3. A particle moves along a straight line such that its distance s(t) metres from a fixed point at time t is given by s(t) =te 2t. Find the velocity and acceleration.

6 6 7.3 Differential Equations An equation that contains derivatives is called a differential equation. Here are two examples: (a) (b) dx =2x y d 2 x 10x =0x dt2 Notation: We often use y to represent. Also when the derivative is with respect to dx time we sometimes use ẋ and ẍ to represent dx dt and d2 x respectively. So we could write dt 2 (a) and (b) as (a) y =2x y (b) ẍ 10x =0. Order: Differential equations can be classified by their order. The order of a differential equation is the degree of the highest derivative that occurs in that equation. So in (a) above the differential equation has order 1 and in (b) it has order 2. Dependent and Independent Variables: In a function such as y = x 3 2x we say that x is the independent variable and y is the dependent variable, since the value of y depends on the choice of value we make for x. In the differential equation =2x yit is x that is the independent variable and y is dx the dependent variable. In the differential equation d2 x 10x = 0 it is t that is the independent variable and x is dt2 the dependent variable. Note: in differential equations the dependent variable is always the variable being differentiated (on the top) and we differentiate with respect to the independent variable (on the bottom). Linear Differential Equations: A differential equation is said to be linear if the dependent variable and its derivatives occur to the first power only and if there are no products involving the dependent variable and its derivatives. The linearity of a differential equation is not affected by the presence of non-linear terms involving the independent variable. So if y is the dependent variable and x the independent variable, linear differential equations will contain terms like y, dx, d2 y dx 2,x2 y, e x. A differential equation that is not dx linear is said to be non-linear and will contain terms like y 2, cos y, y ( ) d 2 2 dx, y. dx 2 Linear: sin x dx + y = x, d 2 x dt 2 + dx dt x t4 =0

7 7 Non-linear: d + sin y = x, dx 2 x dt 2 + xdx dt t4 =0 Constant Coefficient Linear Differential Equations: A linear differential equation has constant coefficients if the coefficients of the dependent variable and its derivatives are constants. So 5 d2 y dx 8 +2y= sin x 2 dx is linear differential equation with constant coefficients. Whereas 5 d2 y dx x +2y= sin x 2 dx is a linear differential equation but it does not have constant coefficients (since the coefficient of is x which is not a constant). dx Exercises State the order of the following differential equations and state which is the dependent and which is the independent variable. (a) d2 y dx y = 2 dx, dt (b) T = 10 α, dα (c) d2 θ dt 2dθ 2 dt =10θ 2. State whether the following differential equations are linear. (a) d2 y dx 2 y dx =0, dx (b) dt = 10 tet, (c) dx dt = x2 7.4 First Order Differential Equations Many scientific laws are formulated in terms of differential equations. In later courses you will learn how to solve such equations. In this module we will be stuing their formulation and checking that given expressions satisfy particular differential equations. First order differential equations involve one derivative dx or y. Consider the differential equation dx =3 Since the gradient is constant (equal to 3 ) the solution is the straight line y =3x+C where C is a constant. If we have extra information such as y = 7 when x = 1 we can determine the value of C. In general, solutions of first order differential equations involve one arbitrary constant. Example 7.6 The velocity y(t) of a particle moving in a resisting medium is decreasing at a rate proportional to y 2 (t). If k>0 is the constant of proportionality, obtain the differential equation satisfied by y(t).

8 8 Solution The rate of decrease of y(t) is dt so dt = ky2. Hence dt = ky2. Example 7.7 In a model to estimate the depreciation of the value of a computer, the value y(t) at age t months, decreases at a rate proportional to y(t). (a) If k>0 is the constant of proportionality, obtain the differential equation satisfied by y(t). (b) The initial value of the computer is Verify that the solution is y(t) = 1000e kt. Solution (a) The rate of decrease of y(t) is dt (b) We have to Show that y(t) = 1000e kt satisfies dt = ky. Check that for y(t) = 1000e kt, y(0) = so dt = ky. Hence dt = ky. For y(t) = 1000e kt, dt = 1000ke kt and ky(t) = 1000ke kt. Hence dt = ky. Putting t =0 in y(t) = 1000e kt gives y(0) = 1000e 0 = 1000 as required. Example 7.8 Verify that y =1+Ce 3t is a solution of dt =3(y 1) where C is a constant. Hence find the solution with y =5 when t =0. Solution Left Hand Side (LHS) = dt =3Ce3t. RHS = 3(y 1) = 3(1 + Ce 3t 1) = 3Ce 3t. Hence LHS = RHS and so /dt =3(y 1) as required. Putting t =0 in y=1+ce 3t gives so C = 4 and y =1+4e 3t. 5=1+Ce 0 =1+C Example 7.9 In a certain chemical reaction, the concentration y(t) of a substance is decreasing at a rate proportional to the cube of its value at that instant. If k>0 is the constant of proportionality, obtain the differential equation satisfied by y(t). If the initial concentration is y(0)=1, verify that the solution is Find T such that y(t )=1/2. y(t) =(2kt +1) 1/2

9 9 Solution The rate of decrease of y(t) is dt For y(t)=(2kt +1) 1/2 so dt = ky3. Hence dt = ky3. LHS = dt =(2k)( 1/2)(2kt +1) 3/2 = k(2kt +1) 3/2 and the RHS = ky 3 = k(2kt +1) 3/2 so LHS = RHS so dt = ky3. Putting t =0 in y(t)=(2kt +1) 1/2 gives y(0) = (1) 1/2 = 1 as required. If y(t )=1/2 1 (2kT +1) = 1 1/2 2 Then (2kT +1) 1/2 = 2 and 2kT + 1 = 4 so that T = 3 2k Exercises In a certain chemical reaction, the concentration y(t) of a substance is deceasing at a rate proportional to the square of its value at that instant. If k>0 is the constant of proportionality, obtain the differential equation satisfied by y(t). 2. Verify that y =2+ C is a solution of t + y = 2 where C is a constant. t dt Hence find the solution with y = 7 when t =3. 3. The velocity y(t) of a particle moving in a resisting medium satisfies the differential equation dt = t y Verify that the solution with y(0) = 5 is y(t) =1 t+4e t. 4. The height y(t) of a tank of water which is being drained satisfies the differential equation y (t) = 16 y(t). Verify that the solution with y(0) = 1 is y(t) =(1 8t) In an electric circuit, the current y(t) satisfies the differential equation dt = E y where the applied voltage E is a constant. Verify that the solution with y(0) = 0 is y(t) =E(1 e t ). What is the value of the current for large t? 6. A hard-boiled egg is put in a basin of water whose temperature is 18 C. The temperature of the egg at time t is y(t), and y(t) is decreasing at a rate proportional to y(t) 18. If k>0 is the constant of proportionality, obtain the differential equation satisfied by y(t). Verify that y(t) = 18 + Ce kt is a solution where C is a constant. Hence find y(t) if the initial temperature of the egg is 98 C.

10 10 7. Verify that y = t ln t + Ct is a solution of t dt = t + y where C is a constant. Hence find the solution with y = 5 when t = Second Order Differential Equations Second order differential equations are equations involving the derivatives y. Such equations occur in many areas of science and engineering. The equation d 2 y dt + 2 k2 y =0 where k is a constant arises in vibrational problems. Equations such as y + ay + by =0 where a and b are constants arise in the analysis of electric circuits and in mechanics problems. The differential equation y = 0 has the solution y(t) =A+Bt where A and B are constants. In general, for second order differential equations we need two pieces of information to find the two arbitrary constants in a solution. Example 7.10 Show that y(t) = Asin 3t + B cos 3t satisfies the differential equation d 2 y dt +9y=0 2 where A and B are constants. Hence find the solution with y(0) = 5 and y (0) = 6. Solution y =3Acos 3t 3B sin 3t and y = 9A sin 3t 9B cos 3t. Then LHS = d2 y +9y= 9Asin 3t 9B cos 3t +9Asin 3t +9Bcos 3t =0 dt2 The RHS = 0 and so LHS = RHS as required. Putting t =0 in y(t)=asin 3t + B cos 3t gives 5 =Asin 0 + B cos 0 = B so B = 5. Putting t =0 in y (t)=3acos 3t 3B sin 3t gives 6 = 3A so A =2. Hence y(t) = 2 sin 3t 5 cos 3t. Example 7.11 Show that y(t) =(A+Bt)e t satisfies the differential equation y 2y + y =0 where A and B are constants. Hence find the solution with y(0) = 3 and y (0)=1.

11 11 Solution Using the product rule, y = Be t +(A+Bt)e t =(A+B+Bt)e t y = Be t +(A+B+Bt)e t =(A+2B+Bt)e t Therefore LHS = y 2y + y =(A+2B+Bt)e t 2(A + B + Bt)e t +(A+Bt)e t =0 The RHS = 0 and so LHS = RHS as required. Putting t =0 in y(t)=(a+bt)e t gives A =3. Putting t =0 in y =(A+B+Bt)e t gives 1 = A + B =3+B. Hence B = 2 and y(t) =3e t 2te t. Example 7.12 Find the value of A if y(t) =Ae 2t is a solution of y +5y +4y =6e 2t. Solution y = 2Ae 2t and y =4Ae 2t. Then LHS y +5y +4y=4Ae 2t 10Ae 2t +4Ae 2t = 2Ae 2t = RHS if A = 3 Example 7.13 Verify that y(t) =e mt is a solution of y +6y +5y=0 if m 2 +6m+5=0. Hence obtain two solutions of the above differential equation. Solution y(t) =e mt so y = me mt and y = m 2 e mt. Then y +6y +5y=m 2 e mt +6me mt +5e mt =(m 2 +6m+5)e mt =0 if m 2 +6m+ 5 = 0. To find two solutions we solve m 2 +6m+5=(m+ 5)(m +1)=0 to get m = 5 and m = 1. The solutions of the differential equation are y(t) =e 5t and y(t) =e t. Exercises Find the value of A if y(t) =Ae 3t is a solution of y 6y +3y=12e 3t. 2. Show that y(t) =Ae 4t + Be 4t satisfies the differential equation d 2 y 16y =0 dt2 where A and B are constants. Hence find the solution with y(0) = 0 and y (0)= Show that y(t) =Asin 2t + B cos 2t sin 4t satisfies the differential equation d 2 y +4y= 12 sin 4t dt2

12 12 where A and B are constants. Hence find the solution with y(0) = 5 and y (0)=8. 4. Verify that y(t) =e mt is a solution of y 36y =0 if m 2 36 = 0. Hence obtain two solutions of the above differential equation. 5. Verify that y(t) =e mt is a solution of y + y 6y =0 if m 2 + m 6 = 0. Hence obtain two solutions of the above differential equation. 6. Find the value of A if y(t) =Asin 2t is a solution of y + y = 21 sin 2t 7.6 Answers to Exercises Exercises by7 2. 5by5by5 3. 4by4by by 10 by Exercises 7.2 1(a) 7 1(b) 12 2(a) 8 2(b) 10 2(c) v(t) =e 2t 2te 2t,a(t)=4te 2t 4e 2t Exercises (a) 2nd order, x independent, y dependent. (b) 1st order, α independent, T dependent. (c) 2nd order, t independent, θ dependent. 2. (a) non-linear, (b) linear, (c) non-linear Exercises y = ky 2 2. y =2+ 15 t 5. E 6. = k(y 18). dt 7. t ln t +5t Exercises A = e 4t 3e 4t e kt 3. y(t) = 6 sin 2t + 5 cos 2t sin 4t 4. e 6t and e 6t 5. e 3t and e 2t 6. A = 7

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