Question

select a suitable identity and find the following products
$(7d-9e)(7d-9e)$

Answer 1

$(7d-9e)(7d-9e)$

$={(7d-9e)}^2$

$={\left(7d\right)}^2+{\left(9e\right)}^2-2\left(7d\right)\left(9e\right)[\text{Using identity:}{(a-b)}^2=a^2+b^2-2ab]$

$=49d^2+81e^2-126de$