Multiply using the rule for the square of a binomial:
Recognise the pattern. The expression is a single binomial raised to the second power, so the shortcut applies. Matching terms gives and .
Square the first term. . Both the coefficient and the power get squared — a common slip is to write .
Double the product of the two terms. . Because the binomial has a minus sign, this middle term is subtracted.
Square the last term. . Squaring removes the sign, so the constant is positive even though the original term was .
Assemble the trinomial. Putting the three pieces together in descending powers gives .
Check with one value. At the original is and the expansion is , so the two agree.
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