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Question

In a right triangle ABC, right-angled at B, D is a point on hypotenuse such that BDAC. If DPAB and DQBC then prove that

(a) DQ2=DP.QC

(b) DP2=DQ.AP

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Solution

We know that if a perpendicular is drawn from the vertex of the right angle of a right triangle to the hypotenuse then the triangles on the both sides of the perpendicular are similar to the whole triangle and also to each other.
(a) Now using the same property in In △BDC, we get

△CQD ∼ △DQB
fraction numerator C Q over denominator D Q end fraction equals fraction numerator D Q over denominator Q B end fraction rightwards double arrow D Q squared equals Q B cross times C Q

Now. since all the angles in quadrilateral BQDP are right angles.
Hence, BQDP is a rectangle.
So, QB = DP and DQ = PB

D Q squared equals D P cross times C Q

(b)

Similarly, △APD ∼ △DPB
fraction numerator A P over denominator D P end fraction equals fraction numerator P D over denominator P B end fraction rightwards double arrow D P squared equals A P cross times P B rightwards double arrow D P squared equals A P cross times D Q
[∵DQ=PB]

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