please help c3 question, deduce smallest value for theta for... (Rcos question)

https://drive.google.com/file/d/0Bzs0gPAC3oXiQTQ4MGNhS2pibzA/view
hey! I have no idea how to do number 1 part C!! I did a and b with no probem. Thank youuuu

You appear to have done part (c) anyway - what was the problem?

You appear to have done part (c) anyway - what was the problem?

Consider the difference between the expansions of cos(A+B) and cos(A-B). Or you could just start again and change the left-hand-side of the c) equation into RCos form and you'll probably realise as you're doing this that it's very similar to part a).