2013 CEMC Gauss (Grade 8) Problems/Problem 17

Problem

$PQRS$ is a rectangle with diagonals $PR$ and $QS$ , as shown. The value of y is

$\textbf{(A)}\ 30 \qquad \textbf{(B)}\ 40 \qquad \textbf{(C)}\ 45 \qquad \textbf{(D)}\ 50 \qquad \textbf{(E)}\ 60$

~diagram uploaded by grogg007

The interior angles of a rectangle are all right angles, and the acute angles of a right triangle sum up to $90^{\circ}$ . Thus, we have the following equations:

$5x^{\circ} + 4x^{\circ} = 90^{\circ}$

$4x^{\circ} + y^{\circ} = 90^{\circ}$

Solving the first equation for $x$ , we get:

$9x^{\circ} = 90^{\circ}$

$x = 10$

Plugging $x$ into the second equation, we have:

$4 \times 10^{\circ} + y^{\circ} = 90^{\circ}$

$40^{\circ} + y^{\circ} = 90^{\circ}$

$y = \boxed {\textbf {(D) } 50}$

~anabel.disher

We can use the above process to find $x$ , and then notice $y$ and $5x$ would be alternate interior angles. Thus,

$y = 5x = 5 \times 10 = \boxed {\textbf {(D) } 50}$

~anabel.disher

We can also get to the conclusion that $y = 5x$ by using the equations:

$4x = 90 - 5x$

$4x + y = 90 - 5x + y = 90$

$y = 90 - 90 + 5x = 5x = 5 \times 10 = \boxed {\textbf {(D) } 50}$

~anabel.disher

2013 CEMC Gauss (Grade 8) (Problems • Answer Key • Resources)
Preceded by Problem 16	Followed by Problem 18
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CEMC Gauss (Grade 8)