On these problems they are just testing your knowledge that every 2Pi you are back to 0 (a full revolution). So the best thing to do is see how many 2Pies you can take out.
Problem can be rewritten as : Cos (Pi + 150Pi + Pi/2)
Since 150 Pi is = 75 (2Ppies) you can take that out of the equation.
Now you have Cos (Pi + Pi/2)
Which equals Cos (3Pi /2 )
Which equals 0
The way to do this one is to calculate the reference angle. In this case the reference angle is to (Pi+150Pi+.5Pi)the 150 Pi is a reference angle of zero so you can ignore that.Therefore the question is basically asking for the cos (Pi+.5Pi)=Cos(180+90)=Cos(270)=0