Maths Olympiad Prep

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, 2012

Geometry Difficulty 3.0 AMC 10/12 Prove it Canada

Quadrilateral PQRSPQRS is constructed with QR=51QR=51, as shown. The diagonals of PQRSPQRS intersect at 9090^{\circ} at point TT, such that PT=32PT=32 and QT=24QT=24.

Figure 0

Calculate the length of PQPQ.
Calculate the area of PQR\triangle PQR.
If QS:PR=12:11QS:PR=12:11, determine the perimeter of quadrilateral PQRSPQRS.

Solution

In PTQ\triangle PTQ, PTQ=90\angle PTQ=90^{\circ}.

Using the Pythagorean Theorem, PQ2=322+242PQ^2=32^2+24^2 or PQ2=1024+576=1600PQ^2=1024+576=1600 and so PQ=1600=40PQ=\sqrt{1600}=40, since PQ gt;0\text{PQ gt;0}.
In QTR\triangle QTR, QTR=90\angle QTR=90^{\circ}.

Using the Pythagorean Theorem, 512=TR2+24251^2=TR^2+24^2 or TR2=2601576=2025TR^2=2601-576=2025 and so TR=2025=45TR=\sqrt{2025}=45, since TR gt;0\text{TR gt;0}.

Since PT=32PT=32 and TR=45TR=45, then PR=PT+TR=32+45=77PR=PT+TR=32+45=77.

In PQR\triangle PQR, QTQT is perpendicular to base PRPR and so PQR\triangle PQR has area12×(PR)×(QT)=12×77×24=924\frac{1}{2}\times(PR)\times(QT)=\frac{1}{2}\times77\times24=924.
From part (b), the length of PRPR is 77.

Since QS:PR=12:11QS:PR=12:11, then QS77=1211\dfrac{QS}{77}=\dfrac{12}{11} or QS=77×1211=84QS=77\times\dfrac{12}{11}=84.

So then TS=QSQT=8424=60TS=QS-QT=84-24=60.

Using the Pythagorean Theorem in PTS\triangle PTS, PS=322+602PS=\sqrt{32^2+60^2} or PS=4624=68PS=\sqrt{4624}=68, since PS gt;0\text{PS gt;0}.

Using the Pythagorean Theorem in RTS\triangle RTS, RS=452+602RS=\sqrt{45^2+60^2} or RS=5625=75RS=\sqrt{5625}=75, since RS gt;0\text{RS gt;0}.

Thus, quadrilateral PQRSPQRS has perimeter 40+51+75+6840+51+75+68 or 234.

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