(3) x³+ax²-x²-a
=(x³-x²)+(ax²-a)
=x²(x-1)+a(x²-1)
=x²(x-1)+a(x+1)(x-1)
=(x-1){x²+a(x+1)}
=(x-1)(x²+ax+a)
(4) 6x²+7xy+2y²+x-2
=6x²+(7y+1)x+(2y²-2)
=6x²+(7y+1)x+2(y²-1)
=6x²+(7y+1)x+2(y+1)(y-1)
たすき掛け
2 (y-1) → 3(y-1)
3 2(y+1) → 2×2(y+1)
={2x+(y-1)}{3x+2(y+1)}
=(2x+y-1)(3x+2y+2)
