Question

已知f（x）是一次函数,且f（f（x））=8x+7.求f（x）.

Answer 1

f(x)=ax+b
f(f(x))=a(ax+b)+b=8x+7
a^2x+ab+b=8x+7
a^2=8,a=2√2,-2√2
ab+b=7,b=-1-2√2,-1+2√2
:.f(x)=2√2x-1+2√2
或f(x)=-2√2x-1-2√2