cos(a+pi/4), если sin a=0,8
cosa=sqrt(1-0.64)=sqrt0.36=0.6
cos(a+pi/4)=0.6*sqrt2/2-0.8*sqrt2/2=3sqrt2/10-2sqrt2/5=3sqrt2/10-4sqrt2/10=-sqrt2/10
ответ cos(a+pi/4)=-sqrt2/10
cos(a+pi/4), если sin a=0,8
cosa=sqrt(1-0.64)=sqrt0.36=0.6
cos(a+pi/4)=0.6*sqrt2/2-0.8*sqrt2/2=3sqrt2/10-2sqrt2/5=3sqrt2/10-4sqrt2/10=-sqrt2/10
ответ cos(a+pi/4)=-sqrt2/10