用配湊法比較簡單.
f(x²+1)+f(x²)=2x⁴+4
=(x²+1)²+(x²)²-2x²-1+4
=(x²+1)²+(x²)²-(x²+1)-x² +2+2
=[(x²+1)²-(x²+1)+2]+[(x²)²-x²+2]
f(x²+1)=(x²+1)²-(x²+1)+2
f(x²)=(x²)²-x²+2
f(x)=x²-x+2
f(x)的表達式為f(x)=x²-x+2
此種方法可以避免繁瑣的計算.