(a+d+c-d)(a-b+c+d)=?

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(a+d+c-d)(a-b+c+d)=?
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(a+d+c-d)(a-b+c+d)=?
(a+d+c-d)(a-b+c+d)=?

(a+d+c-d)(a-b+c+d)=?
(a+b+c-d)(a-b+c+d)
=[(a+c)+(b-d)][(a+c)-(b-d)]
=(a+c)²-(b-d)²
=a²+c²-2ac-b²-d²+2bd

(a+d+c-d)(a-b+c+d)
=[(a+c)+(b-d)] *[(a+c)-(b-d)]
=(a+c)²-(b-d)²

由题可知a=2,b=9.则原式=2的2次方-9的2次方=4-81=-79