(1) 若f(x)=ax+b,则f(X1+X2/2)=f(X1)+f(X2)/2
f(x1+x2/2)=a(x1+x2)/2+b
f(x1)+f(x2)=ax1+b+ax2+b=a(x1+x2)+2b
f(x1)+f(x2)/2=a(x1+x2)/2+b
所以f(X1+X2/2)=f(X1)+f(X2)/2
(2) 若g(x)=X2+ax+b,则g(x1+x2/2)