Question

умоляю помогите ))))))

Answer 1

сos²a=1:(1+tg²a)=1:(1+9/16=1:25/16=16/25
cosa=-4/5
sina=√(1-cos²a)=√(1-16/25)=√9/25=3/5
cos(π/4+a)=cosπ/4*cosa-sinπ/4*sina=√2/2*(-4/5)-√2/2*3/5=
=√2/2*(-4/5-3/5)=√2/2*(-7/5)=-7√2/10