Question

Докажите справедливость равенства.

Answer 1

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Answer 2

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