Need help on a cos/sin explanation problem!

Explain why the value of [cos$\displaystyle theta$, sin$\displaystyle theta$) *dot* [(cos(90+$\displaystyle theta$), sin(90+$\displaystyle theta$)] is independent of $\displaystyle theta$.

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^^ this is one of the problems I have due tomorrow. Problem is I don't even know what it's asking. independent? Are they saying that if you dot those, you can remove the theta and it still works? If this is what its asking I have no idea. i've been thinking about this for like half an hour and i think I'm missing something key? Please help =(