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(m+n)(m-n)=m2-mn+nm-n2=m2-n2
(q-p)(q+p)=q2-p2
(c-d)(c+d)=c2-d2
(a-c)(a+c)=a2-c2
(x+y)(x-y)=x2-y2
(y-5)(y+5)=y2-25
(x+2)(2-x)=4-x2
(1-a)(1+a=1-a2
(n-2m)(n+2m)=n2-4m2
(2x-3y)(2x+3y)=4x2-9y2
(8a+9b)(9b-8a-=81b2-64a2
(5x+3y)(3y-5x)=9y2-25x2