1.=a(b+c)-(b+c)^2=(a-b-c)(b+c) 2.=2a(a+2b)-3c(a+2b)=(2a-3c)(a+2b)
3.=6(x-5y)(x+2y)=-1 4.=(2x-y)^2+(x-1)^2=0,所以x=1,y=2x=2,所求值=1
5.a+bc+b+ca=(a+b)(c+1)=8,a+b=4,c+1=2,周长=5,或a+b=2,c+1=4.(舍)或c+1=1(舍)或c+1=8(舍)
6.(2k+1)^2-(2k-1)^2=8k
1+2+……+n=[(3^2-1^2)+(5^2-3^2)+……+((2n+1)^2-(2n-1)^2)]/8=[(2n+1)^2-1^2]/8=n(n+1)/2