Use the sumtoproduct formula to simplify the expression If s

Use the sum-to-product formula to simplify the expression. If sin39 degree + sin21 degree = sin A degree, 0

By identify,

sinA + SinB = 2 sin ((A+B)/2) cos ((A+B)/2)

sin39°+ sin21° = 2 sin((39+21°)/2)cos((39°-21°)/2)

= 2sin30°cos9°
=2*1/2*cos9°
=cos9°
Now

SinA° = 2sin (30°) cos (9°)

sinA° = 2 (1/2) cos (9°)

sinA° = cos (9°)

A° = arcsin (0.9876)

A=81°