1/a-x+1/a+x-2x/a²+x²-4x³/a的四次方-x四次方
= 1/a-x+1/a+x-2x/a²+x²-4x³/a^4-x^4
= 1/a+1/a-x+x-2x/a²+x²-4x³/a^4-x^4
= 2/a-2x/a²+x²-4x³/a^4-x^4