I have to show that $P, R, Q, S $ are on a circle.











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Let $ABCD $ a paralelogram and $H $ the hortocenter of $triangle ABC $.



Let $PQ $ , $RS $ trough $H $ s.t. $PQ|| AB $ and $RS||BC $ and $Pin [DA], Rin [AB], Qin [BC] , Sin [CD] $.



I have to show that $P, R, Q, S $ are on a circle.



I tried to prove that $m (angle P)+ m (angle R)=180$.










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    Let $ABCD $ a paralelogram and $H $ the hortocenter of $triangle ABC $.



    Let $PQ $ , $RS $ trough $H $ s.t. $PQ|| AB $ and $RS||BC $ and $Pin [DA], Rin [AB], Qin [BC] , Sin [CD] $.



    I have to show that $P, R, Q, S $ are on a circle.



    I tried to prove that $m (angle P)+ m (angle R)=180$.










    share|cite|improve this question


























      up vote
      1
      down vote

      favorite









      up vote
      1
      down vote

      favorite











      Let $ABCD $ a paralelogram and $H $ the hortocenter of $triangle ABC $.



      Let $PQ $ , $RS $ trough $H $ s.t. $PQ|| AB $ and $RS||BC $ and $Pin [DA], Rin [AB], Qin [BC] , Sin [CD] $.



      I have to show that $P, R, Q, S $ are on a circle.



      I tried to prove that $m (angle P)+ m (angle R)=180$.










      share|cite|improve this question















      Let $ABCD $ a paralelogram and $H $ the hortocenter of $triangle ABC $.



      Let $PQ $ , $RS $ trough $H $ s.t. $PQ|| AB $ and $RS||BC $ and $Pin [DA], Rin [AB], Qin [BC] , Sin [CD] $.



      I have to show that $P, R, Q, S $ are on a circle.



      I tried to prove that $m (angle P)+ m (angle R)=180$.







      geometry euclidean-geometry circle quadrilateral






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      edited Nov 16 at 12:37









      Batominovski

      31.8k23189




      31.8k23189










      asked Nov 16 at 11:58









      rafa

      544212




      544212






















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          If $angle ABC=frac{pi}{2}$, then $ABCD$ is a rectangle, and $H=B$. Clearly, we then have $P=A$, $S=C$, and $R=Q=B$, so the points $P$, $Q$, $R$, and $S$ do indeed lie on a circle. If $angle ACB=frac{pi}{2}$, then $H=C$, so $P=D$, $Q=S=C$, and $R=B$. Thus, $P$, $Q$, $R$, and $S$ are again concyclic. If $angle BAC=frac{pi}{2}$, then $H=A$, and so $P=R=A$, $Q=B$, and $S=D$. That is, $P$, $Q$, $R$, and $S$ lie on a single circle. From now on, we assume that $ABC$ is not a right triangle, so that $Hnotin{A,B,C}$.



          Project $H$ (orthogonally) onto $BC$ and $AB$ at $X$ and $Y$, respectively. Note that the triangles $HAP$, $HXQ$, $HCS$, and $HXR$ are similar triangles. That is,
          $$frac{HP}{HS}=frac{HA}{HC}text{ and }frac{HQ}{HR}=frac{HX}{HY},.$$
          Thus,
          $$frac{HPcdot HQ}{HScdot HR}=frac{HAcdot HX}{HCcdot HY},.$$
          As $angle AXC=angle HXC=frac{pi}{2}=angle HYA=angle CYA$, the quadrilateral $AYXC$ is cyclic, so
          $$HCcdot HX=HCcdot HY$$
          by the Power-of-Point Theorem. This means
          $$frac{HPcdot HQ}{HScdot HR}=frac{HAcdot HX}{HCcdot HY}=1,,$$
          or $$HPcdot HQ=HScdot HR,.$$
          By the converse of the Power-of-Point Theorem, we conclude that $PRQS$ is a cyclic quadrilateral.






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            If $angle ABC=frac{pi}{2}$, then $ABCD$ is a rectangle, and $H=B$. Clearly, we then have $P=A$, $S=C$, and $R=Q=B$, so the points $P$, $Q$, $R$, and $S$ do indeed lie on a circle. If $angle ACB=frac{pi}{2}$, then $H=C$, so $P=D$, $Q=S=C$, and $R=B$. Thus, $P$, $Q$, $R$, and $S$ are again concyclic. If $angle BAC=frac{pi}{2}$, then $H=A$, and so $P=R=A$, $Q=B$, and $S=D$. That is, $P$, $Q$, $R$, and $S$ lie on a single circle. From now on, we assume that $ABC$ is not a right triangle, so that $Hnotin{A,B,C}$.



            Project $H$ (orthogonally) onto $BC$ and $AB$ at $X$ and $Y$, respectively. Note that the triangles $HAP$, $HXQ$, $HCS$, and $HXR$ are similar triangles. That is,
            $$frac{HP}{HS}=frac{HA}{HC}text{ and }frac{HQ}{HR}=frac{HX}{HY},.$$
            Thus,
            $$frac{HPcdot HQ}{HScdot HR}=frac{HAcdot HX}{HCcdot HY},.$$
            As $angle AXC=angle HXC=frac{pi}{2}=angle HYA=angle CYA$, the quadrilateral $AYXC$ is cyclic, so
            $$HCcdot HX=HCcdot HY$$
            by the Power-of-Point Theorem. This means
            $$frac{HPcdot HQ}{HScdot HR}=frac{HAcdot HX}{HCcdot HY}=1,,$$
            or $$HPcdot HQ=HScdot HR,.$$
            By the converse of the Power-of-Point Theorem, we conclude that $PRQS$ is a cyclic quadrilateral.






            share|cite|improve this answer



























              up vote
              1
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              accepted










              If $angle ABC=frac{pi}{2}$, then $ABCD$ is a rectangle, and $H=B$. Clearly, we then have $P=A$, $S=C$, and $R=Q=B$, so the points $P$, $Q$, $R$, and $S$ do indeed lie on a circle. If $angle ACB=frac{pi}{2}$, then $H=C$, so $P=D$, $Q=S=C$, and $R=B$. Thus, $P$, $Q$, $R$, and $S$ are again concyclic. If $angle BAC=frac{pi}{2}$, then $H=A$, and so $P=R=A$, $Q=B$, and $S=D$. That is, $P$, $Q$, $R$, and $S$ lie on a single circle. From now on, we assume that $ABC$ is not a right triangle, so that $Hnotin{A,B,C}$.



              Project $H$ (orthogonally) onto $BC$ and $AB$ at $X$ and $Y$, respectively. Note that the triangles $HAP$, $HXQ$, $HCS$, and $HXR$ are similar triangles. That is,
              $$frac{HP}{HS}=frac{HA}{HC}text{ and }frac{HQ}{HR}=frac{HX}{HY},.$$
              Thus,
              $$frac{HPcdot HQ}{HScdot HR}=frac{HAcdot HX}{HCcdot HY},.$$
              As $angle AXC=angle HXC=frac{pi}{2}=angle HYA=angle CYA$, the quadrilateral $AYXC$ is cyclic, so
              $$HCcdot HX=HCcdot HY$$
              by the Power-of-Point Theorem. This means
              $$frac{HPcdot HQ}{HScdot HR}=frac{HAcdot HX}{HCcdot HY}=1,,$$
              or $$HPcdot HQ=HScdot HR,.$$
              By the converse of the Power-of-Point Theorem, we conclude that $PRQS$ is a cyclic quadrilateral.






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                up vote
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                If $angle ABC=frac{pi}{2}$, then $ABCD$ is a rectangle, and $H=B$. Clearly, we then have $P=A$, $S=C$, and $R=Q=B$, so the points $P$, $Q$, $R$, and $S$ do indeed lie on a circle. If $angle ACB=frac{pi}{2}$, then $H=C$, so $P=D$, $Q=S=C$, and $R=B$. Thus, $P$, $Q$, $R$, and $S$ are again concyclic. If $angle BAC=frac{pi}{2}$, then $H=A$, and so $P=R=A$, $Q=B$, and $S=D$. That is, $P$, $Q$, $R$, and $S$ lie on a single circle. From now on, we assume that $ABC$ is not a right triangle, so that $Hnotin{A,B,C}$.



                Project $H$ (orthogonally) onto $BC$ and $AB$ at $X$ and $Y$, respectively. Note that the triangles $HAP$, $HXQ$, $HCS$, and $HXR$ are similar triangles. That is,
                $$frac{HP}{HS}=frac{HA}{HC}text{ and }frac{HQ}{HR}=frac{HX}{HY},.$$
                Thus,
                $$frac{HPcdot HQ}{HScdot HR}=frac{HAcdot HX}{HCcdot HY},.$$
                As $angle AXC=angle HXC=frac{pi}{2}=angle HYA=angle CYA$, the quadrilateral $AYXC$ is cyclic, so
                $$HCcdot HX=HCcdot HY$$
                by the Power-of-Point Theorem. This means
                $$frac{HPcdot HQ}{HScdot HR}=frac{HAcdot HX}{HCcdot HY}=1,,$$
                or $$HPcdot HQ=HScdot HR,.$$
                By the converse of the Power-of-Point Theorem, we conclude that $PRQS$ is a cyclic quadrilateral.






                share|cite|improve this answer














                If $angle ABC=frac{pi}{2}$, then $ABCD$ is a rectangle, and $H=B$. Clearly, we then have $P=A$, $S=C$, and $R=Q=B$, so the points $P$, $Q$, $R$, and $S$ do indeed lie on a circle. If $angle ACB=frac{pi}{2}$, then $H=C$, so $P=D$, $Q=S=C$, and $R=B$. Thus, $P$, $Q$, $R$, and $S$ are again concyclic. If $angle BAC=frac{pi}{2}$, then $H=A$, and so $P=R=A$, $Q=B$, and $S=D$. That is, $P$, $Q$, $R$, and $S$ lie on a single circle. From now on, we assume that $ABC$ is not a right triangle, so that $Hnotin{A,B,C}$.



                Project $H$ (orthogonally) onto $BC$ and $AB$ at $X$ and $Y$, respectively. Note that the triangles $HAP$, $HXQ$, $HCS$, and $HXR$ are similar triangles. That is,
                $$frac{HP}{HS}=frac{HA}{HC}text{ and }frac{HQ}{HR}=frac{HX}{HY},.$$
                Thus,
                $$frac{HPcdot HQ}{HScdot HR}=frac{HAcdot HX}{HCcdot HY},.$$
                As $angle AXC=angle HXC=frac{pi}{2}=angle HYA=angle CYA$, the quadrilateral $AYXC$ is cyclic, so
                $$HCcdot HX=HCcdot HY$$
                by the Power-of-Point Theorem. This means
                $$frac{HPcdot HQ}{HScdot HR}=frac{HAcdot HX}{HCcdot HY}=1,,$$
                or $$HPcdot HQ=HScdot HR,.$$
                By the converse of the Power-of-Point Theorem, we conclude that $PRQS$ is a cyclic quadrilateral.







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                edited Nov 16 at 12:43

























                answered Nov 16 at 12:36









                Batominovski

                31.8k23189




                31.8k23189






























                     

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