1/2x³-1/4x+0.2x²+0.25x-0.5x³-1/5x²
=(1/2x³-0.5x³)+(0.2x²-1/5x²)+(-1/4x+0.25x)
=0
∵无论x取何值,代数式的值都等于0
∴代数式的值与x无关.