If 1a2 2 x + b2 2 x dx = 112 -1(3 x) + constant, then the maximum value of a x + b x, is :
- 41
- 39
- 40
- 42
Answer
(3) 40
Solution
Divide by 2 x: I = 2 x dxa2 2 x + b2; put x = t
I = dta2t2 + b2 = 1ab -1 ( ab x ) + C
Compare: ab = 12, ab = 3 a2 = 36, b2 = 4
maximum of a x + b x is a2 + b2
maximum = 36 + 4 = 40
Answer: (3)