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June 22, 2020

Side Length of Rectangle | AMC-10A, 2009 | Problem 17

Try this beautiful problem from Geometry based on Side Length of Rectangle.

Side Length of Rectangle - AMC-10A, 2009- Problem 17

Rectangle $A B C D$ has $A B=4$ and $B C=3 .$ Segment $E F$ is constructed through $B$ so that $E F$ isperpendicular to $D B$, and $A$ and $C$ lie on $D E$ and $D F$, respectively. What is $E F$ ?

  • $9$
  • $10$
  • $\frac{125}{12}$
  • \(\frac{103}{9}\)
  • \(12\)

Key Concepts




Check the Answer

Answer: $\frac{125}{12}$

AMC-10A (2009) Problem 10

Pre College Mathematics

Try with Hints

Finding Side Length of Rectangle

We have to find out the length of \(EF\)

Now $BD$ is the altitude from $B$ to $EF$, we can use the equation $BD^2 = EB\cdot BF$. ( as \(\triangle BDE \sim \triangle BDF\)).so we have to find out \(BE\) and \(BF\)

Can you now finish the problem ..........

Problem figure

Now Clearly, $\triangle BDE \sim \triangle DCB$. Because of this, $\frac{A B}{C B}=\frac{E B}{D B}$. From the given information and the Pythagorean theorem, $A B=4, C B=3$, and $D B=5 .$ Solving gives $E B=20 / 3$
We can use the above formula to solve for $B F . B D^{2}=20 / 3 \cdot B F$. Solve to obtain $B F=15 / 4$

can you finish the problem........

Problem figure

Therefore $E F=E B+B F=\frac{20}{3}+\frac{15}{4}=\frac{80+45}{12}$

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One comment on “Side Length of Rectangle | AMC-10A, 2009 | Problem 17”

  1. This problem can be done using coordinate geometry by using convenient coordinate system. Let us consider $D$ to be the origin. Then by your diagram it follows that the coordinate of $A,B$ and $C$ are respectively $(-0,-3),(4,-3)$ and $(4,0).$ Then the equation of the line joining $B$ and $D$ is $3x+4y=0.$ So the equation of the line $L$ passing through $B$ and perpendicular to $BD$ is given by $4x - 3y = 25.$ So coordinate of $F$ and $E$ can be found by the point of intersections of $L$ with $X$-axis and the point of intersection of $L$ with $Y$-axis respectively. Therefore the coordinate of $E$ and $F$ are respectively $\left (0, -\frac {25} {3} \right)$ and $\left (\frac {25} {4} , 0 \right ).$ So the length of $EF$ is $$\sqrt {\left (-\frac {25} {4} \right )^2 + \left (- \frac {25} {3} \right )^2} = \frac {125} {12}.$$

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