Question

Докажите справедливость равенства.

Answer 1

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Answer 2

$\mathtt{sin51аcos39а-sin21аcos9а=}\\\\\mathtt{\frac{sin(51а-39а)+sin(51а+39а)}{2}-\frac{sin(21а-9а)+sin(21а+9а)}{2}=}\\\\\mathtt{\frac{sin12а+sin90а-sin12а-sin30а}{2}=\frac{sin90а-sin30а}{2}=\frac{1-\frac{1}{2}}{2}=\frac{\frac{1}{2}}{2}=\frac{1}{4}}$