Straight line

Submitted by alpha on Mon, 11/02/2009 - 05:10.

Geometry questions

If the segments joining the points A(a,b)and B(c,d) subtends an angle theta at the origin, prove that $cos \theta=\frac{ac+bd}{\sqrt{(a^2+b^2)(c^2+d^2)}}$ .

What would be the diagram?

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Straight line