When multiplying two powers that have the same base ($\sin\left(a\right)-1$), you can add the exponents
$\left(\sin\left(a\right)-1\right)^2$
Intermediate steps
2
A binomial squared (difference) is equal to the square of the first term, minus the double product of the first by the second, plus the square of the second term. In other words: $(a-b)^2=a^2-2ab+b^2$
$\sin\left(a\right)^2-2\sin\left(a\right)+1$
Final Answer
$\sin\left(a\right)^2-2\sin\left(a\right)+1$
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