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If O is the origin and if the coordina...

If `O` is the origin and if the coordinates of any two points `Q_1 \ a n d \ Q_2` are `(x_1,y_1) \ a n d \ (x_2,y_2),` respectively, prove that `O Q_1.O Q_2cos/_Q_1O Q_2=x_1x_2+y_1y_2`.

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