Question

if sina+sinb=-21/65,cosa+cosb=-27/65 then cos(a-b/2)=?

Answer 1

Sin a + Sin b = 2 Sin (a + b)/2 . Cos (a-b)/2

Cos a + Cos b = 2 Cos (a + b)/2. Cos ( a-b)/2

square them and add them

4 sin² ( a + b)/2 . Cos² (a-b) /2 + 4 Cos² (a + b)/2 . Cos² (a- b)/2

4 Cos² (a-b)/2 [ Sin² (a + b)/2 + Cos² (a + b)/2] = 4 Cos² (a-b)/2 . 1

since we squared and added. We have to square the values and add them too.

(-21/65)² + (-27/65)² = 4 Cos² (a-b)/2

(1170/4225) = 4 Cos ² (a-b)/2

now you can find the value by doing the last few steps right??

Answer 2

@vanimanasa1998

Answer 3

thank u...