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**scorpion007** Two particles R and S move so that their position vector, r and s respectively, are given by:

r = (2-2cos(t)) i + (1+sin(t)) j and s = sin(t) i + 2cos(t) j

where t seconds (t > 0) is the time elapsed since the start of their motions.

a) Evaluate r (dot) s and hence deduce the exact time when the position vectors of the two particles are first at right angles.

Now, what i got so far is sin(t) + cos(t) = 0, but im not too sure where to go from there, or if i am even in the right ballpark.

Any help appreciated.