Question

Помогите решить задание хелп

Answer 1

$sina= sqrt{1-( frac{4}{5} )^2} = sqrt{1- frac{16}{25} } = sqrt{ frac{9}{25} } = frac{3}{5}$

$tga= frac{sina}{cosa}= frac{ frac{3}{5} }{ frac{4}{5} } = frac{3}{4}$

$cos2a=1-2sin^2a=1-2*( frac{3}{5} )^2=1- 2*frac{9}{25} =1- frac{18}{25} = frac{25-18}{25} = frac{7}{25}$

$cos13+cos77=2cos frac{13+77}{2} cos frac{13-77}{2} =2cos frac{90}{2} cos frac{-64}{2} =2cos45cos32=$$2* frac{ sqrt{2} }{2} cos32= sqrt{2}cos32$