Given:
5f(x)+4f(x1)=x1+3....(1)
Replace x→x1
5f(x1)+4f(x)=x+3....(2)
Eliminating f(x1) from (1)&(2), we get
9f(x)=x5+15−4x−12
⇒9f(x)=x5−4x+3
⇒18f(x)=x10−8x+6
⇒18∫12f(x)dx=∫12(x10−8x+6)dx
⇒18∫12f(x)dx=[10logx−4x2+6x]12
⇒18∫12f(x)dx=[(10log2−4)−(10log1+2)]
⇒18∫12f(x)dx=10loge2−6
Hence this is the correct option.