Question

if x-y=2 and 2/x+y = 1/5 then​

Answer 1

Sin[(B+C)/2]

Since A+B+C=180 for interior angles of triangle ABC.

then B+C=180-A.

NOW   Sin [(180-A)/2]

             =Sin[90-(A/2)]                      since Sin(90-A)=CosA

              =Cos(A/2)