中二數學20分

3a(a+b)-(a-b)(a+b)
=(a+b)(3a-a+b)
=(a+b)(2a+b)

(4m-n)(2m+n)+(7n-m)(2m+n)
=(2m+n)(4m-n+7n-m)
=(2m+n)(3m+6n)
= 2(2m+n)(m+2n)

1/a = 1/b - 1/c
bc =ac - ab----------(*abc)
ab=ac-bc
ab=c(a-b)
c=ab/a-b

8x ³ y² -2ox²y²+4xy²
=4xy²(2x²y-5x+1)
=4*3*-1
=-12

(x²+3x)²+3(x²+3x)+2
=(x²+3x)²+(x²+3x)+2(x²+3x)+2
=(x²+3x)(x²+3x+1)+2(x²+3x+1)
=(x²+3x+2)(x²+3x+1)
=(x²+x+2x+2)(x²+3x+1)
=(x²+3x+1)(x+1)(x+2)