sin(3a)- cos(2a)=0

Question

cos(2a)=sin(90-2a)

 

Answer 1

Work out the right side.

 

sin(90 - 2a) =

 

sin(90)cos(2a) - cos(90)sin(2a) =

 

cos(2a)

 

 

Also, notice that 2a and (90 - 2a) are complementary to each other.  sine and cosine are equal to each other under this complementary condition.