Q. 194.1( 15 Votes )

In a right triangle ABC, right-angled at B, D is a point on hypotenuse such that BD AC. If DP AB and DQ BC then prove that (a) DQ2 = DP • QC (b) DP2 = DQ • AP.


Answer :

By the property that says, if a perpendicular is drawn from the vertex of a right triangle to the hypotenuse then the triangles on both the sides of the perpendicular are similar to the whole triangle and also to each other.

We can conclude by the property in ∆BDC,


∆CQD ∆DQB


(a). To Prove: DQ2 = DP × QC


Proof: As already proved, ∆CQD ∆DQB


We can write the ratios as,



By cross-multiplication, we get


DQ2 = QB × QC …(i)


Now since, quadrilateral PDQB forms a rectangle as all angles are 90° in PDQB.


DP = QB & PB = DQ


And thus replacing QB by DP in equation (i), we get


DQ2 = DP × QC


Hence, proved.


(b). To Prove: DP2 = DQ × AP


Prof: Similarly using same property, we get


∆APD ∆DPB


We can write the ratios as,



By cross-multiplication, we get


DP2 = PB × AP


DP2 = DQ × AP [ PB = DQ]


Hence, proved.


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