trigo (3)

Using formula
cosA + cosB = 2 cos[(A + B)/2] cos[(A - B)/2]

L.H.S.
= cos(2a+b)+cosb
= 2 cos[(2a + b + b)/2] cos[(2a + b - b)/2]
= 2 cos[(2a + 2b)/2] cos[2a/2]
= 2 cos(a + b) cos a
= 2cosacos(a+b)
= R.H.S.