Let f be a twice differentiable function such that f(x)= 0x (t-x) dt- 0x f(t) t dt, x (- 2, 2 ). Then f'' ( 6 )+12f' (- 6 )+f ( 6 ) is equal to .
Answer
5
Solution
Put u=t-x: 0x (t-x) dt= -x0 u du= [- e u ]-x0= e x
f(x)= e x- 0x f(t) t dt, and f(0)=0
Differentiating: f'(x)=- x-f(x) x
f'(x)+f(x) x=- x (linear), I.F. =e x dx= x
f(x) x=- x x dx=- x+C
f(0)=0 0=-1+C C=1
f(x)= x-1, f'(x)=- x, f''(x)=- x
f'' ( 6 )+12f' (- 6 )+f ( 6 )=- 32+12 12+ 32-1
=5
Answer: 5