16-49m² =(4+7m)(4-7m)
-四分之一x×y^3+0.09xy=-xy[ (1/4)y² - 0.09]=(y/2 + 0.3)(y/2 - 0.3)
-1+16x² = (4x+1)(4x-1)
a²b^3-4a²b=a²b(b+2)(b-2)
6.4²-3.6² =(6.4+3.6)(6.4-3.6)=10 X 2.8 = 28
1.4²×9-2.3²×36 = 1.4²×3²-2.3²×6²=(1.4×3-2.3×6)(1.4×3+2.3×6)=(4.2-13.8)(4.2+13.8)=-9.6 X 18 =-172.8
xy-9x^3y=xy(1+3x)(1-3x)
a^3[x+y]²-4a^3c² =a³（3x+3y+2c）（3x+3y-2c）
4a²[x-y]+b²[y-x] =(x-y)(2a+b)(2a-b)
4[a-b]²-[a+b]²=(2a-2b+a+b)(2a-2b-a-b)=(3a-b)(a-3b)