[tex](-a-b)^2=(-(a+b))^2=(a+b)^2\\(-a-b)(a+b)=-a^2-ab-ab-b^2=-\left ( a^2+ab+ab+b^2 \right )=-(a+b)^2\\(a+b-c)^2=(a+b)^2-2(a+b)c+c^2=a^2+2ab+b^2-2ac-2bc+c^2=\\=a^2+c^2+c^2+2ab-2ac-2bc\\(a-b-c)^2=(a-b)^2-2(a-b)c+c^2=a^2-2ab+b^2-2ac+2bc=\\=a^2+b^2+c^2-2ab-2ac+2bc[/tex]
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[tex](-a-b)^2=(-(a+b))^2=(a+b)^2\\(-a-b)(a+b)=-a^2-ab-ab-b^2=-\left ( a^2+ab+ab+b^2 \right )=-(a+b)^2\\(a+b-c)^2=(a+b)^2-2(a+b)c+c^2=a^2+2ab+b^2-2ac-2bc+c^2=\\=a^2+c^2+c^2+2ab-2ac-2bc\\(a-b-c)^2=(a-b)^2-2(a-b)c+c^2=a^2-2ab+b^2-2ac+2bc=\\=a^2+b^2+c^2-2ab-2ac+2bc[/tex]