Question

Помогите решить ........................

Answer 1

$3)sqrt{2}Cos(alpha -beta)=sqrt{2}(Cosalpha Cosbeta+Sinalpha Sinbeta)=sqrt{2}( frac{1}{2}-frac{1}{2})=0\\4)3Cos(alpha+beta)=3(Cosalpha Cosbeta-Sinalpha Sinbeta)=3(-frac{1}{2}+frac{1}{2})=0$