(x-1)6=a6x6+a5x5+a4x4+a3x3+a2x2+a1x+a0
a1为一次项系数
T6=C(6,5)x^1*(-1)^5=-6x
a1=-6
a2为x^2项系数
T5=C(6,4)x^2(-1)^4=15x^2
a2=15