\(\left(a+b-c\right)^2-\left(a-c\right)^2-2ab-2bc\)
\(=\left(a+b\right)^2-2.\left(a+b\right).c+c^2-a^2+2ac-c^2-2ab-2bc\)
\(=a^2+2ab+b^2-2ac-2bc+c^2-a^2+2ac-c^2-2ab-2bc\)
\(=b^2-4bc\)
\(=b\left(b-4c\right)\)
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(a+b-c)2-(a-c)2-2ab-2bc
=a2+ b2+ c2+ 2ab- 2bc- 2ca- a2+ 2ac- c2- 2ab- 2bc
=b2-4bc
=b.(b-4c)
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