(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)
(Q) ∴2f(x)f(y)=f(x−y)+f(x+y)
Replacing x by y and y by x , then
2f(y)f(x)=f(y−x)+f(y+x)
From equation (1) and (2), we get
f(x−y)=f(y−x) Put
y=0, then differentiate
(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=nan−1+nbn−1