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( x + y )^2 = x^2 + 2xy + y^2  definitely NOT x^2 + y^2

Take a close look at what's different here:

( xy )^2

These are being multiplied...
We can use the rule:

( xy )^2 = ( x^2 )( y^2 )

( x + y )^2

These are being added...
We CANNOT use the
rule!

answers