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In the given figure, PQRS is a parallelogram with PS = 7 cm, PT = 4 cm and PV = 5 cm. What is the length of RS in cm? (The diagram is representative.)

Parallelogram PQRS with PS = 7 along the top; PT = 4 is perpendicular from P to QR, and PV = 5 is perpendicular from P to RS, meeting it at V

  • A.\(\frac{20}{7}\)
  • B.\(\frac{28}{5}\)
  • C.\(\frac{9}{2}\)
  • D.\(\frac{35}{4}\)

Answer: B

RS is 28/5 cm.

The area of a parallelogram is any side times the perpendicular distance to it from the opposite side. PT is the height on base QR, and PV is the height on base RS.

\[ \begin{aligned} QR \times PT &= RS \times PV \\ 7 \times 4 &= RS \times 5 \\ RS &= \frac{28}{5} = 5.6\ \text{cm} \end{aligned} \]

QR equals PS = 7 cm because opposite sides of a parallelogram are equal. 20/7 is PT times PV divided by PS, which mixes the two heights, and 35/4 is PS times PV over PT, which pairs each height with the wrong base.

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