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解一元二次方程十字相乘法教案(十字相乘分解因式法典型例题)
导语:分解因式——十字相乘法
(1):(x+y)(x+y+2xy)+(xy+1)(xy-1)
分析:∵(x+y)和(x+y+2xy)都含有(x+y),故把其看为一个整体展开得(x+y)²+2xy(x+y),而(xy+1)+(xy-1)=2xy,故可用十字相乘法。
解:原式=(x+y)²+2xy(x+y)+(xy+1)(xy-1)
=(x+y+xy+1)(x+y+xy-1)
=(x+1)(y+1)(x+y+xy-1)
注:
x+y xy+1
x+y xy-1
(2):x³+(2a+1)x²+(a²+2a-1)x+(a²-1)
解:原式=x³+(2a+1)x²+(a²+a)x+(a-1)x+(a²-1)
=x[x²+(2a+1)x+a(a+1)]+(a-1)(x+a+1)
=x(x+a)(x+a+1)+(a-1)(x+a+1)
=(x+a+1)(x²+ax+a-1)
=(x+1)(x+a-1)(x+a+1)
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