Please help with this question:

The ellipse x^2/a^2 + y^2/b^2 = 1 cuts the x axis at A and B. There is a vertical tangent to the ellipse at A and also at B. The tangent to the ellipse at the point P (a cos t, b sin t) cuts the tangents through A and B at Q and R respectively. Prove that dAQ.dBR = b^2

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