Let I(x) = 3 dx(4x + 6) ( 4x2 + 8x + 3 ) and I(0) = 34 + 20. If I ( 12 ) = a 2b + c, where a, b, c N, (a, b) = 1, then a + b + c is equal to :
- 30
- 29
- 28
- 31
Answer
(4) 31
Solution
Here 4x2 + 8x + 3 = (2x + 1)(2x + 3) and 4x + 6 = 2(2x + 3).
Put t = 2x + 3, so dx = dt2 and 2x + 1 = t - 2:
I = 3 dt22t t(t - 2) = 34 dtt2 1 - 2t
Now put 1 - 2t = u2, so 2t2 dt = 2u du, i.e. dtt2 = u du:
I = 34 u duu = 34u + C
Hence I(x) = 34 2x + 12x + 3 + C and I(0) = 34 1 3 + C = 34 + C.
I(0) = 34 + 20, we get C = 20, so
I ( 12 ) = 34 24 + 20 = 34 22 + 20 = 3 28 + 20
Thus a = 3, b = 8, c = 20 (and (3, 8) = 1).
a + b + c = 3 + 8 + 20 = 31
Answer: (4)