Let f(x)+2f ( 1x )=x2+5 and 2g(x)-3g ( 12 )=x, x>0. If = 12 f(x) dx, and = 12 g(x) dx, then the value of 9 + is :
- 0
- 10
- 1
- 11
Answer
(4) 11
Solution
f(x)+2f ( 1x )=x2+5 ... (i)
Replace x by 1x: f ( 1x )+2f(x)= 1x2+5 ... (ii)
2 (ii) - (i): 3f(x)= 2x2-x2+5
f(x)= 13 ( 2x2-x2+5 )
= 13 [- 2x- x33+5x ]12= 13 [ (-1- 83+10 )- (-2- 13+5 ) ]
= 13 ( 193- 83 )= 119 9 =11
For g, put x= 12: 2g ( 12 )-3g ( 12 )= 12 g ( 12 )=- 12
2g(x)+ 32=x g(x)= x2- 34
= [ x24- 3x4 ]12= (1- 32 )- ( 14- 34 )=0
9 + =11+0=11
Answer: (4)