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05 SQUARE ROOTS AND CUBE ROOTS EXAMPLES
Ex. 1. Evaluate √6084 by factorization method .
Sol. Method: Express the given number as the product of prime factors.
Now, take the product of these prime factors choosing one out of
every pair of the same primes. This product gives the square root
of the given number.
| 2 | 6084 |
| 2 | 3042 |
| 3 | 1521 |
| 3 | 507 |
| 13 | 169 |
| 13 |
\ √6084 = (2 x 3 x 13) = 78.
Thus, resolving 6084 into prime factors, we get:
6084 = 22 x 32 x 132
Ex. 2. Find the square root of 1471369.
Sol. Explanation: In the given number, mark off the digits 1 1471369 (1213
in pairs starting from the unit's digit. Each pair and 1
the remaining one digit is called a period. 22 47
Now, 12 = 1. On subtracting, we get 0 as remainder. 44
Now, bring down the next period i.e., 47. 241 313
Now, trial divisor is 1 x 2 = 2 and trial dividend is 47. 241
So, we take 22 as divisor and put 2 as quotient. 2423 7269
The remainder is 3. 7269
Next, we bring down the next period which is 13. x
Now, trial divisor is 12 x 2 = 24 and trial dividend is
313. So, we take 241 as dividend and 1 as quotient.
The remainder is 72.
Bring down the next period i.e., 69.
Now, the trial divisor is 121 x 2 = 242 and the trial
dividend is 7269. So, we take 3as quotient and 2423
as divisor. The remainder is then zero.
Hence, √1471369 = 1213.
Ex. 3. Evaluate: √248 + √51 + √ 169 .
Sol. Given expression = √248 + √51 + 13 = √248 + √64 = √ 248 + 8 = √256 = 16.
Ex. 4. If a * b * c = √(a + 2)(b + 3) / (c + 1), find the value of 6 * 15 * 3.
Sol. 6 * 15 * 3 = √(6 + 2)(15 + 3) / (3 + 1) = √8 * 18 / 4 = √144 / 4 = 12 / 4 = 3.
Ex. 5. Find the value of √25/16.
Sol. √ 25 / 16 = √ 25 / √ 16 = 5 / 4
Ex. 6. What is the square root of 0.0009?
Sol. √0.0009= √ 9 / 1000 = 3 / 100 = 0.03.
Ex. 7. Evaluate √175.2976.
Sol. Method: We make even number of decimal places 1 175.2976 (13.24
by affixing a zero, if necessary. Now, we mark off 1
periods and extract the square root as shown. 23 75
69
\√175.2976 = 13.24 262 629
524
2644 10576
10576
x
Ex. 8. What will come in place of question mark in each of the following questions?
(i) √32.4 / ? = 2 (ii) √86.49 + √ 5 + ( ? )2 = 12.3.
Sol. (i) Let √32.4 / x = 2. Then, 32.4/x = 4 <=> 4x = 32.4 <=> x = 8.1.
(ii) Let √86.49 + √5 + x2 = 12.3.
Then, 9.3 + √5+x2 = 12.3 <=> √5+x2 = 12.3 - 9.3 = 3
<=> 5 + x2 = 9 <=> x2 = 9 - 5= 4 <=> x = √4 = 2.
Ex.9. Find the value of √ 0.289 / 0.00121.
Sol. √0.289 / 0.00121 = √0.28900/0.00121 = √28900/121 = 170 / 11.
Ex.10. If √1 + (x / 144) = 13 / 12, the find the value of x.
Sol. √1 + (x / 144) = 13 / 12 Þ ( 1 + (x / 144)) = (13 / 12 )2 = 169 / 144
Þx / 144 = (169 / 144) - 1
Þx / 144 = 25/144 Þ x = 25.
Ex. 11. Find the value of √3 up to three places of decimal.
Sol.
1 3.000000 (1.732
1
27 200
189
343 1100
1029
3462 7100
6924 \√3 = 1.732.
Ex. 12. If √3 = 1.732, find the value of √192 - 1 √48 - √75 correct to 3 places
2
of decimal. (S.S.C. 2004)
Sol. √192 - (1 / 2)√48 - √75 = √64 * 3 - (1/2) √ 16 * 3 - √ 25 * 3
=8√3 - (1/2) * 4√3 - 5√3
=3√3 - 2√3 = √3 = 1.732
Ex. 13. Evaluate: √(9.5 * 0.0085 * 18.9) / (0.0017 * 1.9 * 0.021)
Sol. Given exp. = √(9.5 * 0.0085 * 18.9) / (0.0017 * 1.9 * 0.021)
Now, since the sum of decimal places in the numerator and denominator under the radical sign is the same, we remove the decimal.
\ Given exp = √(95 * 85 * 18900) / (17 * 19 * 21) = √ 5 * 5 * 900 = 5 * 30 = 150.
Ex. 14. Simplify: √ [( 12.1 )2 - (8.1)2] / [(0.25)2 + (0.25)(19.95)]
Sol. Given exp. = √ [(12.1 + 8.1)(12.1 - 8.1)] / [(0.25)(0.25 + 19.95)]
=√ (20.2 * 4) /( 0.25 * 20.2) = √ 4 / 0.25 = √400 / 25 = √16 = 4.
Ex. 15. If x = 1 + √2 and y = 1 - √2, find the value of (x2 + y2).
Sol. x2 + y2 = (1 + √2)2 + (1 - √2)2 = 2[(1)2 + (√2)2] = 2 * 3 = 6.
Ex. 16. Evaluate: √0.9 up to 3 places of decimal.
Sol.
9 0.900000(0.948
81
184 900
736
1888 16400
15104 \√0.9 = 0.948
Ex.17. If √15 = 3.88, find the value of √ (5/3).
Sol. √ (5/3) = √(5 * 3) / (3 * 3) = √15 / 3 = 3.88 / 3 = 1.2933…. = 1.293.
Ex. 18. Find the least square number which is exactly divisible by 10,12,15 and 18.
Sol. L.C.M. of 10, 12, 15, 18 = 180. Now, 180 = 2 * 2 * 3 * 3 *5 = 22 * 32 * 5.
To make it a perfect square, it must be multiplied by 5.
\ Required number = (22 * 32 * 52) = 900.
Ex. 19. Find the greatest number of five digits which is a perfect square.
(R.R.B. 1998)
Sol. Greatest number of 5 digits is 99999.
3 99999(316
9
61 99
61
626 3899
3756
143
\ Required number == (99999 - 143) = 99856.
Ex. 20. Find the smallest number that must be added to 1780 to make it a perfect
square.
Sol.
4 1780 (42
16
82 180
164
16
\ Number to be added = (43)2 - 1780 = 1849 - 1780 = 69.
Ex. 21. √2 = 1.4142, find the value of √2 / (2 + √2).
Sol. √2 / (2 + √2) = √2 / (2 + √2) * (2 - √2) / (2 - √2) = (2√2 – 2) / (4 – 2)
= 2(√2 – 1) / 2 = √2 – 1 = 0.4142.
22. If x = (√5 + √3) / (√5 - √3) and y = (√5 - √3) / (√5 + √3), find the value of (x2 + y2).
Sol.
x = [(√5 + √3) / (√5 - √3)] * [(√5 + √3) / (√5 + √3)] = (√5 + √3)2 / (5 - 3)
=(5 + 3 + 2√15) / 2 = 4 + √15.
y = [(√5 - √3) / (√5 + √3)] * [(√5 - √3) / (√5 - √3)] = (√5 - √3)2 / (5 - 3)
=(5 + 3 - 2√15) / 2 = 4 - √15.
\ x2 + y2 = (4 + √15)2 + (4 - √15)2 = 2[(4)2 + (√15)2] = 2 * 31 = 62.
Ex. 23. Find the cube root of 2744.
Sol. Method: Resolve the given number as the product 2 2744
of prime factors and take the product of prime 2 1372
factors, choosing one out of three of the same 2 686
prime factors. Resolving 2744 as the product of 7 343
prime factors, we get: 7 49
7
2744 = 23 x 73.
\ 3√2744= 2 x 7 = 14.
Ex. 24. By what least number 4320 be multiplied to obtain a number which is a perfect cube?
Sol. Clearly, 4320 = 23 * 33 * 22 * 5.
To make it a perfect cube, it must be multiplied by 2 * 52 i.e,50.
