Class 8 Chapter 7: Square Roots & Cube Roots (Lecture Notes)

Transcription

1 Class 8 Chapter 7: Square Roots & Cube Roots (Lecture Notes) SQUARE OF A NUMBER: The Square of a number is that number raised to the power. Eamples: Square of 9 = 9 = 9 9 = 8 Square of 0. = (0.) = (0.) (0.) = 0.04 PERFECT SQUARE: A natural number is called a perfect square, if it is the square of some natural number. Eample: We have =, = 4, = 9 Some Properties of Squares of Numbers. The square of an even number is always an even number. Eample: is even and = 4, which is even.. The square of an odd number is always an odd number. Eample: is odd and = 9, which is odd.. The square of a proper fraction is a proper fraction less than the given fraction. Eample: Square of ( ) = ( ) ( ) = 4 and we see that 4 < 4. The square of a decimal fraction less than is smaller than the given decimal. Eample: 0. < and (0.) = = 0.0 < A number ending in,, 7 or 8 is never a perfect square. Eample: The numbers 7, 4, 567 and 098 end in,, 7 and 8 respectively. So, none of them is a perfect square. 6. A number ending in an odd number of zeros is never a perfect square. Eamples: The numbers 690, and end in one zero, three zeros and five zeros respectively. So, none of them is a perfect square. SQUARE ROOT: The square root of a number is that number which when multiplied by itself gives as the product. We denote the square root of a number by. Eample: Since 7 7 = 49, so 49 = 7, i.e., the square root of 49 is 7.

2 METHODS OF FINDING THE SQUARE ROOTS OF NUMBERS To Find the Square Root of a Given Perfect Square Number Using Prime Factorization Method:. Resolve the given number into prime factors. Make pairs of similar factors. The product of prime factors, chosen one out of every pair, gives the square root of the given number. Eamples: Find Square root of i) 65 and ii) = Hence 65 = 5 5 = = Hence 96 = = 6 Test for a number to be a Perfect Square: A given number is a perfect square, if it can be epressed as the product of pairs of equal factors. Eample: 96 = Hence 96 = = 6 To Find the Square Root of a given number By Division Method. Mark off the digits in pairs starting with the unit digit. Each pair and remaining one digit (if any) is called a period.. Think of the largest number whose square is equal to or just less than the first period. Take this number as the divisor as well as quotient.. Subtract the product of divisor and quotient from first period and bring down the net period to the right of the remainder. This becomes the new dividend. 4. Now, new divisor is obtained by taking twice the quotient and anneing with it a suitable digit which is also taken as the net digit of the quotient, chosen in such a way that the product of new divisor and this digit is equal to or just less than the new dividend. Repeat steps, and 4 till all the periods have been taken up. Now, the quotient so obtained is the required square root of the given number.

3 Eample: Find the square root of SQUARE ROOT OF NUMBERS IN DECIMAL FORM Method: Make the number of decimal places even, by affiing a zero, if necessary. Now, mark periods (starting from the right most digit) and find out the square root by the long-division method. Put the decimal point in the square root as soon as the integral part is ehausted. Eample: Find the square root of J = 4.86 Square root of numbers which are not perfect squares Eample: Find the value of up to places of decimal

4 = = 0.75 SQUARE ROOTS OF FRACTIONS: For any positive real numbers a and b, we have: i. ab = a b Eample: Find the square root of 44 = ii. a b = a b Eample: 9 6 = 9 6 = 4 CUBE OF A NUMBER: The cube of a number is that number raised to the power. Eample: Cube of = = = 8 PERFECT CUBE: A natural number is said to be a perfect cube, if it is the cube of some natural number. Eample: =, = 8, = 7 and so on CUBE ROOT: The cube root of a number is that number which when multiplied by itself three times gives as the product. We denote the cube root of a number by Eample: Since = 5, therefore 5 METHOD OF FINDING THE CUBE ROOT OF NUMBERS: Cube Root of a Given Number by Prime Factorization Method. Resolve the given number into prime factors.. Make groups in triplets of similar factors.. The product of prime factors, chosen one out of ever triplet, gives the cube root of the given number. Eample: Find Cube of = Therefore 7576 = = 6 Test for a Number to be a Perfect Cube A given natural number is a perfect cube if it can be epressed as the product of triplets of equal factors. Cube Roots of Fractions and Decimals = 5 4

5 Eample: Find cube root of: i ii = 97 = = = 9686 = SQUARE ROOTS BY USING TABLES A table showing the square roots of all natural numbers from to 00 has been given, each approimating to places of decimal using this table, we can find the square roots of numbers, larger than 00, as illustrated in the following eamples Eamples: 0 = 4.47; 54 = 7.48; 8 = 9.0; 99 =

6 CUBE ROOTS OF NUMBERS, USING CUBE ROOT TABLE The table given below shows the values of where is a natural number. Using this table, we may find the cube root of any given natural number Eample: 5 =.70; 50 =.684; 500 = 7.97