If x=arctan(-3/4) and x+y=210 degrees, then cosy=?

Question

If x=arctan(-3/4) and x+y=210 degrees, then cosy=? (Estimate answer to two decimal places).

Answer 1

arctan(-3/4) means to find the angle x that makes tanx=-3/4.  Knowing that,

 

cosy = cos[210 - arctan(-3/4)]

 

 

Use the calculator to find the estimation.