(P) f′(x)=2x+g′(1),f′′(x)=2g′(x)=2x+f′(2),g′′(x)=2 at x=1,f′(1)=2+g′(1) and g′(1)=2+f′(2)⇒f′(1)=4+f′(2)\text { (Q) } \therefore 2 \mathrm{f}(\mathrm{x}) \mathrm{f}(\mathrm{y})=\mathrm{f}(\mathrm{x}-\mathrm{y})+\mathrm{f}(\mathrm{x}+\mathrm{y})
Replacingxbyyandybyx,then2 f(y) f(x)=f(y-x)+f(y+x)
Fromequation(1)and(2),wegetf(x-y)=f(y-x)
Puty=0
,thendifferentiate(R)f′(x)=3x2+2xf′(1)+f′′(2)⇒f′(1)′=3+2f′(1)+f′′(2)⇒f′(1)+f′′(2)=−3⇒ and f′′(x)=6x+2f′(1)∴f′′(2)=12+2f′(1)∴−2f′(1)+f′′(2)=12(S)n(a+b)^{n-1}=n a^{n-1}+n b^{n-1}$