Trig 4-3

It becomes  $-2ab$  since  $\cos90\degree=1$  

it becomes  $+2ab$ since  $\cos90\degree=-1$  

It becomes $0$  since  $\cos90\degree=0$  

It becomes  $-ab$  since  $\cos90\degree=\frac{1}{2}$  

 $A=30\degree,\ B=60\degree,\ C=90\degree$  

 $A=60\degree,\ B=60\degree,\ C=60\degree$  

 $A=45\degree,\ B=45\degree,\ C=90\degree$  

 $A=75\degree,\ B=25\degree,\ C=100\degree$