例1、下列各式中由等号的左边到右边的变形,是因式分解的是( )
A.(x+3)(x-3)=x2-9
B.x2+x-5=(x-2)(x+3)+1
C.a2b+ab2=ab(a+b)
D.
答案:C
例2、把下列各式分解因式.
①4a2-8ab;
②-6x4y2+12x3y-27x2y3;
③5(x-y)3+10(y-x)2;
④25-16x2;
⑤9(m+n)2-(m-n)2;
⑥a5-a;
⑦x2+14x+49;
⑧(m+n)2-6(m+n)+9;
⑨-3ax2+6axy-3ay2;
⑩(x2+4)2+8x(x2+4)+16x2;
(x+y)2-4(x+y-1);
(x4+1)2-(2x2)2;
(x+y)2+4(x-y)2-4(x2-y2).
解:
①原式=4a(a-2b).
②原式=-3x2y(2x2y-4x+9y2).
③原式=5(x-y)3+10(x-y)2=5(x-y)2(x-y+2).
④原式=52-(4x)2=(5+4x)(5-4x).
⑤原式=[3(m+n)]2-(m-n)2=(3m+3n+m-n)(3m+3n-m+n)
=(4m+2n)(2m+4n)
=4(2m+n)(m+2n).
⑥原式=a(a4-1)=a(a2+1)(a2-1)=a(a2+1)(a+1)(a-1).
⑦原式=x2+2×7x+72=(x+7)2.
⑧原式=(m+n)2-2×3(m+n)+32=(m+n-3)2.
⑨原式=-3a(x2-2xy+y2)=-3a(x-y)2.
⑩原式=(x2+4)2+2×4x·(x2+4)+(4x)2
=(x2+4+4x)2=[(x+2)2]2=(x+2)4.
原式=(x+y)2-4(x+y)+4=(x+y-2)2.
原式=(x4+1+2x2)(x4+1-2x2)=(x2+1)2(x2-1)2
=(x2+1)2(x+1)2(x-1)2.
原式=(x+y)2-2(x+y)·2(x-y)+[2(x-y)]2
=[x+y-2(x-y)]2=(3y-x)2.
例3、分解因式:m2-n2+2m-2n.
解:
原式=(m2-n2)+2(m-n)=(m-n)(m+n)+2(m-n)
=(m-n)(m+n+2).
变式:分解因式:
①4a3-4a2-a+1;
②a2-b2+2b-1.
解:
①原式=(4a3-4a2)-(a-1)=4a2(a-1)-(a-1)
=(a-1)(4a2-1)=(a-1)(2a+1)(2a-1).
②原式=a2-(b2-2b+1)=a2-(b-1)2=(a+b-1)(a-b+1).
例4、分解因式:x2+4x+3.
解:原式=(x2+4x+4)-1=(x+2)2-1=(x+2+1)(x+2-1)=(x+3)(x+1).
变式:分解因式:x3-3x2+4.
解:原式=x3+x2-4x2+4
=x2(x+1)-4(x+1)(x-1)
=(x+1)(x2-4x+4)=(x+1)(x-2)2.
例5、分解因式:
(x+y)(x+y+2xy)+(xy+1)·(xy-1).
解:
设x+y=a,xy=b,则
原式=a(a+2b)+(b+1)(b-1)
=a2+2ab+b2-1=(a+b)2-1=(a+b+1)(a+b-1)
=(x+y+xy+1)(x+y+xy-1)
=[x(1+y)+(y+1)](x+y+xy-1)
=(x+1)(y+1)(x+y+xy-1).