2001 AMC 10 Problems/Problem 24


In trapezoid $ABCD$, $\overline{AB}$ and $\overline{CD}$ are perpendicular to $\overline{AD}$, with $AB+CD=BC$, $AB<CD$, and $AD=7$. What is $AB\cdot CD$?

$\textbf{(A)}\ 12 \qquad \textbf{(B)}\ 12.25 \qquad \textbf{(C)}\ 12.5 \qquad \textbf{(D)}\ 12.75 \qquad \textbf{(E)}\ 13$


[asy]  /* Geogebra to Asymptote conversion, documentation at artofproblemsolving.com/Wiki, go to User:Azjps/geogebra */ import graph; size(7cm);  real labelscalefactor = 0.5; /* changes label-to-point distance */ pen dps = linewidth(0.7) + fontsize(10); defaultpen(dps); /* default pen style */  pen dotstyle = black; /* point style */  real xmin = -4.3, xmax = 7.3, ymin = -3.16, ymax = 6.3; /* image dimensions */  /* draw figures */ draw(circle((0.2,4.92), 1.3));  draw(circle((1.04,1.58), 2.14));  draw((-1.1,4.92)--(0.2,4.92));  draw((0.2,4.92)--(1.04,1.58));  draw((1.04,1.58)--(-1.1,1.58));  draw((-1.1,1.58)--(-1.1,4.92));  /* dots and labels */ dot((-1.1,4.92),dotstyle);  label("$A$", (-1.02,5.12), NE * labelscalefactor);  dot((0.2,4.92),dotstyle);  label("$B$", (0.28,5.12), NE * labelscalefactor);  dot((-1.1,1.58),dotstyle);  label("$D$", (-1.02,1.78), NE * labelscalefactor);  dot((1.04,1.58),dotstyle);  label("$C$", (1.12,1.78), NE * labelscalefactor);  clip((xmin,ymin)--(xmin,ymax)--(xmax,ymax)--(xmax,ymin)--cycle);  /* end of picture */ [/asy]

If $AB=x$ and $CD=y$, then $BC=x+y$. By the Pythagorean theorem, we have $(x+y)^2=(y-x)^2+49.$ Solving the equation, we get $4xy=49 \implies xy = \boxed{\textbf{(B)}\ 12.25}$.

Solution 2

Simpler is just drawing the trapezoid and then using what is given to solve. Draw a line parallel to $\overline{AD}$ that connects the longer side to the corner of the shorter side. Name the bottom part $x$ and top part $a$. By the Pythagorean theorem, it is obvious that $a^{2} + 49 = (2x+a)^{2}$ (the RHS is the fact the two sides added together equals that). Then, we get $a^2 + 49 = 4x^2 + 4ax + a^2$, cancel out and factor and we get $49 = 4x(x+a)$. Notice that $x(x+a)$ is what the question asks, so the answer is $\boxed{\textbf{(B)}\ 12.25}$.

Solution by IronicNinja

Solution 3

We know it is a trapezoid and that $\overline{AB}$ and $\overline{CD}$ are perpendicular to $\overline{AD}$. If they are perpendicular to $\overline{AD}$ that means this is a right-angle trapezoid (search it up if you don't know what it looks like or you can look at the trapezoid in the first solution). We know $\overline{AD}$ is $7$. We can then set the length of $\overline{AB}$ to be $x$ and the length of $\overline{DC}$ to be $y$. $\overline{BC}$ would then be $x+y$. Let's draw a straight line down from point $B$ which is perpendicular to $\overline{DC}$ and parallel to $\overline{AD}$. Let's name this line $M$. Then let's name the point at which line $M$ intersects $\overline{DC}$ point $E$. Line $M$ partitions the trapezoid into ▭ $ADEB$ and $\triangle$ $BEC$. We will use the triangle to solve for $xy$ using the Pythagorean theorem. The line segment $\overline{EC}$ would be $y-x$ because $\overline{DC}$ is $y$ and $\overline{DE}$ is $x$. $\overline{DE}$ is $x$ because it is parallel to $\overline{AB}$ and both are of equal length. Because of the Pythagorean theorem, we know that $(EC)^2+(BE)^2=(BC)^2$. Substituting the values we have we get $(y-x)^2+(7)^2=(x+y)^2$. Simplifying this we get $(y^2-2xy+x^2)+(49)=(x^2+2xy+y^2)$. Now we get rid of the $x^2$ and $y^2$ terms from both sides to get $(-2xy)+(49)=(2xy)$. Combining like terms we get $(49)=(4xy)$. Then we divide by $4$ to get $(12.25)=(xy)$. Now we know that $xy\ =\ 12.25$ which is answer choice $\boxed{\textbf{(B)}\ 12.25}$.


Fixed $\text{\LaTeX}$ - Mliu630XYZ, palaashgang, anshulb, JoyfulSapling

Solution 4 (EZ Cheez)

Choose any value for $BC$, and then use Pythagorean theorem to get $CD - AB$, and $AB = (BC - (CD - AB))/2$). Then multiply $AB \cdot CD$.

For example:

$BC=25$. $CD - AB = \sqrt{25^2 - 7^2} = 24$. $AB = (25 - 24)/2=0.5$. $CD = 0.5 + 24 = 24.5$. $AB \cdot CD = (0.5)(24.5)= \boxed{\textbf{(B)}\ 12.25}$.

See Also

2001 AMC 10 (ProblemsAnswer KeyResources)
Preceded by
Problem 23
Followed by
Problem 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
All AMC 10 Problems and Solutions

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