Question

1)cos2alpha,якщо sin alpha=4/5

Answer 1

$cos2 alpha =cos^2 alpha -sin^2 alpha cos^2 alpha =1-sin^2 alpha =1-( frac{4}{5})^2=1-( frac{16}{25})= frac{9}{25} cos alpha = frac{3}{5} cos2 alpha = ( frac{3}{5})^2-( frac{4}{5})^2= frac{9}{25}- frac{16}{25} = -frac{7}{25}$