（a+2）（a-2）-（2a-1）²+（a+1）²
=a²-4-(4a²-4a+1)+(a²+2a+1)
=a²-4-4a²+4a-1+a²+2a+1
=-2a²+6a-4
（x-6）（x²+x+1）-x(2x+1)(3x-1)
=x³+x²+x-6x²-6x-6-x(6x²+x-1)
=x³-5x²-5x-6-6x³-x²+x
=-5x³-6x²-4x-6