f:R→R f(x)=acos(x3−x)+b∣x∣sin(∣x3+x)
(A) at a=0&b=1 f(x)=0+∣x∣sin(∣∣x3+x∣∣) ={xsin(x3+x)xsin(x3+x)x<0x≥0
So f(x) differentiable at x=0
(B) at a=1&b=0 f(x)=cos(x3−x) is differentiable at x=1
(C) at a=1&b=0 f(x)=cos(x3−x) is differentiable at x=0
(D) at a=1&b=1 f(x)=cos(x3−x)+xsin(x3+x) is always differentiable