-
If ab = 21 and (a + b)2 = 25 then the value of a2 + b2 + 3ab is (a − b)2 4
-
- 115
- 121
- 125
- 127
Correct Option: B
| = | ||
| (a − b)2 | 4 |
By componendo and dividendo,
| = | ||
| (a + b)2 − (a − b)2 | 25 − 4 |
| ⇒ | = | ||
| 4ab | 21 |
| ⇒ | = | ||
| 2 × 21 | 21 |
⇒ a2 + b2 = 2 × 29 = 58
∴ a2 + b2 + 3ab = 58 + 3 × 21
= 58 + 63 = 121
