Question

ПУсть sina=5/13,  a∈ II четверти найти sin2a,  cos2a

Answer 1

$sin2a=2siaacosa,co2a=cos ^{2}a-sin ^{2}a cosa= - sqrt{1- frac{25}{169} } =- sqrt{ frac{144}{169}$$cosa=- frac{12}{13} , sin2a=2 frac{5}{13} ( frac{ -12}{13} )=- frac{120}{169}$$cos2a=cos ^{2} a-sin ^{2} a, cos2a= frac{144}{169}- frac{25}{169}= frac{119}{169}$