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Question Number 19321 by ajfour last updated on 09/Aug/17

Parallel tangents to a circle at A  and B are cut in the points C and D  by a tangent to the circle at E.  Prove that AD, BC and the line  joining the middle points of AE  and BE are concurrent.

$$\mathrm{Parallel}\:\mathrm{tangents}\:\mathrm{to}\:\mathrm{a}\:\mathrm{circle}\:\mathrm{at}\:\mathrm{A} \\ $$$$\mathrm{and}\:\mathrm{B}\:\mathrm{are}\:\mathrm{cut}\:\mathrm{in}\:\mathrm{the}\:\mathrm{points}\:\mathrm{C}\:\mathrm{and}\:\mathrm{D} \\ $$$$\mathrm{by}\:\mathrm{a}\:\mathrm{tangent}\:\mathrm{to}\:\mathrm{the}\:\mathrm{circle}\:\mathrm{at}\:\mathrm{E}. \\ $$$$\mathrm{Prove}\:\mathrm{that}\:\mathrm{AD},\:\mathrm{BC}\:\mathrm{and}\:\mathrm{the}\:\mathrm{line} \\ $$$$\mathrm{joining}\:\mathrm{the}\:\mathrm{middle}\:\mathrm{points}\:\mathrm{of}\:\mathrm{AE} \\ $$$$\mathrm{and}\:\mathrm{BE}\:\mathrm{are}\:\mathrm{concurrent}. \\ $$

Commented by ajfour last updated on 09/Aug/17

Answered by ajfour last updated on 10/Aug/17

Commented by ajfour last updated on 10/Aug/17

Let B be origin.  A(0,2r)  . Let m=tan θ  C((r/m), 2r)    ;  D(mr,0)  eq. of AD :   y=2r−(2/m)x  eq. of BC:   y=2mx  I (α,β) lies on both, hence    β=2r−((2α)/m)=2αm  ⇒  α(m+(1/m))=r  or  α=((mr)/(1+m^2 ))        β=((2m^2 r)/(1+m^2 )) .  x_E =rsin 2θ =((2mr)/(1+m^2 ))         x_P =x_M =(x_E /2) = ((mr)/(1+m^2 )) = 𝛂 .

$$\mathrm{Let}\:\mathrm{B}\:\mathrm{be}\:\mathrm{origin}. \\ $$$$\mathrm{A}\left(\mathrm{0},\mathrm{2r}\right)\:\:.\:\mathrm{Let}\:\mathrm{m}=\mathrm{tan}\:\theta \\ $$$$\mathrm{C}\left(\frac{\mathrm{r}}{\mathrm{m}},\:\mathrm{2r}\right)\:\:\:\:;\:\:\mathrm{D}\left(\mathrm{mr},\mathrm{0}\right) \\ $$$$\mathrm{eq}.\:\mathrm{of}\:\mathrm{AD}\::\:\:\:\mathrm{y}=\mathrm{2r}−\frac{\mathrm{2}}{\mathrm{m}}\mathrm{x} \\ $$$$\mathrm{eq}.\:\mathrm{of}\:\mathrm{BC}:\:\:\:\mathrm{y}=\mathrm{2mx} \\ $$$$\mathrm{I}\:\left(\alpha,\beta\right)\:\mathrm{lies}\:\mathrm{on}\:\mathrm{both},\:\mathrm{hence} \\ $$$$\:\:\beta=\mathrm{2r}−\frac{\mathrm{2}\alpha}{\mathrm{m}}=\mathrm{2}\alpha\mathrm{m} \\ $$$$\Rightarrow\:\:\alpha\left(\mathrm{m}+\frac{\mathrm{1}}{\mathrm{m}}\right)=\mathrm{r}\:\:\mathrm{or}\:\:\alpha=\frac{\mathrm{mr}}{\mathrm{1}+\mathrm{m}^{\mathrm{2}} }\: \\ $$$$\:\:\:\:\:\beta=\frac{\mathrm{2m}^{\mathrm{2}} \mathrm{r}}{\mathrm{1}+\mathrm{m}^{\mathrm{2}} }\:. \\ $$$$\mathrm{x}_{\mathrm{E}} =\mathrm{rsin}\:\mathrm{2}\theta\:=\frac{\mathrm{2mr}}{\mathrm{1}+\mathrm{m}^{\mathrm{2}} } \\ $$$$\:\:\:\:\:\:\:\mathrm{x}_{\mathrm{P}} =\mathrm{x}_{\mathrm{M}} =\frac{\mathrm{x}_{\mathrm{E}} }{\mathrm{2}}\:=\:\frac{\boldsymbol{\mathrm{mr}}}{\mathrm{1}+\boldsymbol{\mathrm{m}}^{\mathrm{2}} }\:=\:\boldsymbol{\alpha}\:. \\ $$

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