A learner claims $(a-b)^2=a^2-b^2$. Explain the error using expansion, and give a numerical counte

Question

A learner claims $(a-b)^2=a^2-b^2$. Explain the error using expansion, and give a numerical counterexample.

Accepted Answer

Correct expansion: $a^2-2ab+b^2$; for example $a=5,b=2$ gives 9 and 21.

Testing a square identity