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servidopolisxv
Here you go:
$a\left(a\right)+a\left(-b\right)+\left(-b\right)a+\left(-b\right)\left(-b\right)=$
$={a}^{2}-ab-ba+{b}^{2}=$
$={a}^{2}-2ab+{b}^{2}$
###### Not exactly what you’re looking for?
Stella Calderon
You can use the FOIL method:
First multiply the first terms in (a-b)(a-b):
$a\left(a\right)={a}^{2}$
Add the products of the outside and inside terms:
${a}^{2}+a\left(-b\right)+a\left(-b\right)={a}^{2}-2ab$
Finally, add the product of the last terms:
${a}^{2}-2ab+\left(-b\right)\left(-b\right)={a}^{2}-2ab+{b}^{2}$
###### Not exactly what you’re looking for?
RizerMix

You will get $={a}^{2}-2ab+{b}^{2}$