Question

Use suitable identity to solve $\left(7a-9b\right)\left(7a-9b\right)$

Answer 1

$(7a-9b)(7a-9b)=(7a-9b{)}^2$

using the identity $(a-b{)}^2=a^2-2ab-b^2$

$(7a-9b{)}^2=(7a{)}^2-2\times 7a\times 9b+(9b{)}^2$

$(7a-9b)(7a-9b)=49a^2-126ab+81b^2$