Step 4.-Identify the known and desired angles and a rule to connect them

• b=1/2 the bond angle

• A=60, Co-A=30 degrees

• B=60 degrees, Co-B=30 degrees

• Sine of any middle part is equal to the cosine of the opposite parts

• sin(30)= cos(b)*cos(30)

• sin(30)/cos(30)=cos(b) or cos(b)=tan(30)