(5a+6b-7c)^2-(5a-6b+7c)^2

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(5a+6b-7c)^2-(5a-6b+7c)^2(5a+6b-7c)^2-(5a-6b+7c)^2(5a+6b-7c)^2-(5a-6b+7c)^2(5a+6b-7c)^2-(5a-6b+7c)^2

(5a+6b-7c)^2-(5a-6b+7c)^2
(5a+6b-7c)^2-(5a-6b+7c)^2

(5a+6b-7c)^2-(5a-6b+7c)^2
(5a+6b-7c)^2-(5a-6b+7c)^2
=[(5a+6b-7c)-(5a-6b+7c)]×[(5a+6b-7c)+(5a-6b+7c)]
=(12b-14c)×10a
=20a(6b-7c)

(5a+6b-7c)^2-(5a-6b+7c)^2=【(5a+6b-7c)+(5a-6b+7c)】×【(5a+6b-7c)-(5a-6b+7c)】
=10a(12b-14c)=120ab-140ac

没看懂要求什么

提取负号
(5a+6b-7c)^2+(5a+6b-7c)^2
=2(5a+6b-7c)^2

(5a+6b-7c)^2-(5a-6b+7c)^2
=[5a+(6b-7c)]^2-[5a-(6b-7c)]^2
=[5a+(6b-7c)+5a-(6b-7c)][[5a+(6b-7c)+5a-(6b-7c)]
=10a[2(6b-7c)]=120ab-140ac