AIME 2018 I · 第 4 题

Solution (Easiest Law of Cosines)

We apply Law of Cosines on $\angle A$ twice (one from $\triangle ADE$ and one from $\triangle ABC$),

\begin{align*} 12^2 &= 10^2 + 10^2 - 2(10)(10) \cdot \cos{A} \\[5pt] x^2&=x^2+(10-x)^2-2(x)(10-x)\cdot \cos{A} \end{align*}

Solving for $\cos{A}$ in both equations, we get \begin{align*} \cos{A} &= \frac{7}{25} \\ \cos{A} &= \frac{(10-x)^2}{(2)(10-x)(x)} = \frac{10-x}{2x} \end{align*} Setting the two equal, \begin{align*} \frac{10-x}{2x} &= \frac{7}{25} \\[5pt] 250-25x &= 14x \\[5pt] x &= \frac{250}{39}. \end{align*} Therefore, our answer is $\boxed{289}$

Note that this strategy of applying LOC on congruent or supplementary angles is very common. It's also how Stewart's Theorem is derived.

-RootThreeOverTwo, edits by epiconan

Solution 1 (No Trig)

We draw the altitude from $B$ to $\overline{AC}$ to get point $F$. We notice that the triangle's height from $A$ to $\overline{BC}$ is 8 because it is a $3-4-5$ Right Triangle. To find the length of $\overline{BF}$, we let $h$ represent $\overline{BF}$ and set up an equation by finding two ways to express the area. The equation is $(8)(12)=(10)(h)$, which leaves us with $h=9.6$. We then solve for the length $\overline{AF}$, which is done through pythagorean theorm and get $\overline{AF}$ = $2.8$. We can now see that $\triangle AFB$ is a $7-24-25$ Right Triangle. Thus, we set $\overline{AG}$ as $5-$$\tfrac{x}{2}$, and yield that $\overline{AD}$ $=$ $($ $5-$ $\tfrac{x}{2}$ $)$ $($ $\tfrac{25}{7}$ $)$. Now, we can see $x$ = $($ $5-$ $\tfrac{x}{2}$ $)$ $($ $\tfrac{25}{7}$ $)$. Solving this equation, we yield $39x=250$, or $x=$ $\tfrac{250}{39}$. Thus, our final answer is $250+39=\boxed{289}$.

~bluebacon008

Diagram edited by Afly

Solution 2 (Easy Similar Triangles)

We start by adding a few points to the diagram. Call $F$ the midpoint of $AE$, and $G$ the midpoint of $BC$. (Note that $DF$ and $AG$ are altitudes of their respective triangles). We also call $\angle BAC = \theta$. Since triangle $ADE$ is isosceles, $\angle AED = \theta$, and $\angle ADF = \angle EDF = 90 - \theta$. Since $\angle DEA = \theta$, $\angle DEC = 180 - \theta$ and $\angle EDC = \angle ECD = \frac{\theta}{2}$. Since $FDC$ is a right triangle, $\angle FDC = 180 - 90 - \frac{\theta}{2} = \frac{180-m}{2}$.

Since $\angle BAG = \frac{\theta}{2}$ and $\angle ABG = \frac{180-m}{2}$, triangles $ABG$ and $CDF$ are similar by Angle-Angle similarity. Using similar triangle ratios, we have $\frac{AG}{BG} = \frac{CF}{DF}$. $AG = 8$ and $BG = 6$ because there are $2$ $6-8-10$ triangles in the problem. Call $AD = x$. Then $CE = x$, $AE = 10-x$, and $EF = \frac{10-x}{2}$. Thus $CF = x + \frac{10-x}{2}$. Our ratio now becomes $\frac{8}{6} = \frac{x+ \frac{10-x}{2}}{DF}$. Solving for $DF$ gives us $DF = \frac{30+3x}{8}$. Since $DF$ is a height of the triangle $ADE$, $FE^2 + DF^2 = x^2$, or $DF = \sqrt{x^2 - (\frac{10-x}{2})^2}$. Solving the equation $\frac{30+3x}{8} = \sqrt{x^2 - (\frac{10-x}{2})^2}$ gives us $x = \frac{250}{39}$, so our answer is $250+39 = \boxed{289}$.

Solution 3 (Algebra w/ Law of Cosines)

As in the diagram, let $DE = x$. Consider point $G$ on the diagram shown above. Our goal is to be able to perform Pythagorean Theorem on $DG, GC$, and $DC$. Let $GE = \frac{10-x}{2}$. Therefore, it is trivial to see that $GC^2 = \Big(x + \frac{10-x}{2}\Big)^2$ (leave everything squared so that it cancels nicely at the end). By Pythagorean Theorem on Triangle $DGE$, we know that $DG^2 = x^2 - \Big(\frac{10-x}{2}\Big)^2$. Finally, we apply Law of Cosines on Triangle $DBC$. We know that $\cos(\angle DBC) = \frac{3}{5}$. Therefore, we get that $DC^2 = (10-x)^2 + 12^2 - 2(12)(10-x)\frac{3}{5}$. We can now do our final calculation:

$$DG^2 + GC^2 = DC^2 \implies x^2 - \Big(\frac{10-x}{2}\Big)^2 + \Big(x + \frac{10-x}{2}\Big)^2 = (10-x)^2 + 12^2 - 2(12)(10-x)\frac{3}{5}$$ After some quick cleaning up, we get

$$30x = \frac{72}{5} + 100 \implies x = \frac{250}{39}$$ Therefore, our answer is $250+39=\boxed{289}$.

~awesome1st

Solution 4 (Coordinates)

Let $B = (0, 0)$, $C = (12, 0)$, and $A = (6, 8)$. Then, let $x$ be in the interval $0 and parametrically define$D$and$E$as$(6-3x, 8-4x)$and$(12-3x, 4x)$respectively. Note that$AD = 5x$, so$DE = 5x$. This means that

$$\begin{aligned} \sqrt{36+(8x-8)^2} &= 5x\\ 36+(8x-8)^2 &= 25x^2\\ 64x^2-128x+100 &= 25x^2\\ 39x^2-128x+100 &= 0\\ x &= \dfrac{128\pm\sqrt{16384-15600}}{78}\\ x &= \dfrac{100}{78}, 2\\ \end{aligned}$$ However, since $2$ is extraneous by definition, $x=\dfrac{50}{39}\implies AD = \dfrac{250}{39}\implies\boxed{289}$ ~ mathwiz0803

Solution 5 (Law of Cosines)

As shown in the diagram, let $x$ denote $\overline{AD}$. Let us denote the foot of the altitude of $A$ to $\overline{BC}$ as $F$. Note that $\overline{AE}$ can be expressed as $10-x$ and $\triangle{ABF}$ is a $6-8-10$ triangle . Therefore, $\sin(\angle{BAF})=\frac{3}{5}$ and $\cos(\angle{BAF})=\frac{4}{5}$. Before we can proceed with the Law of Cosines, we must determine $\cos(\angle{BAC})=\cos(2\cdot \angle{BAF})=\cos^2(\angle{BAF})-\sin^2(\angle{BAF})=\frac{7}{25}$. Using LOC, we can write the following statement:

$$(\overline{DE})^2=(\overline{AD})^2+\overline{AE}^2-2(\overline{AD})(\overline{AE})\cos(\angle{BAC})\implies$$ $$x^2=x^2+(10-x)^2-2(x)(10-x)\left(\frac{7}{25}\right)\implies$$ $$0=(10-x)^2-\frac{14x}{25}(10-x)\implies$$ $$0=10-x-\frac{14x}{25}\implies$$ $$10=\frac{39x}{25}\implies x=\frac{250}{39}$$ Thus, the desired answer is $\boxed{289}$ ~ blitzkrieg21

Solution 6

In isosceles triangle, draw the altitude from $D$ onto $\overline{AD}$. Let the point of intersection be $X$. Clearly, $AE=10-AD$, and hence $AX=\frac{10-AD}{2}$.

Now, we recognise that the perpendicular from $A$ onto $\overline{AD}$ gives us two $6$-$8$-$10$ triangles. So, we calculate $\sin \angle ABC=\frac{4}{5}$ and $\cos \angle ABC=\frac{3}{5}$

$\angle BAC = 180-2\cdot\angle ABC$. And hence,

$\cos \angle BAC = \cos \angle (180-2\cdot\angle ABC) = -\cos (2\cdot\angle ABC) = \sin^2 \angle ABC - \cos^2 \angle ABC = 2\cos^2 \angle ABC - 1 = \frac{32}{25}-\frac{25}{25}=\frac{7}{25}$

Inspecting $\triangle ADX$ gives us $\cos \angle BAC = \frac{\frac{10-x}{2}}{x} = \frac{10-x}{2x}$ Solving the equation $\frac{10-x}{2x}=\frac{7}{25}$ gives $x= \frac{250}{39} \implies\boxed{289}$

~novus677

Solution 7 (Area into Similar Triangles)

After calling $x=AD=DE=EC$ and $10-x=AE=BD$, we see we have length ratios in terms of $x$, which motivates area ratios. We look at the area of triangle $ADC$ in two ways in order to find $DG$ (perpendicular from $D$ to $AB$), and then use similar triangles to find $x$.

Using area ratios, $[ADC] = \frac{x}{10}\cdot[ABC] = \frac{x}{10} \cdot 48 = \frac{24x}{5}$. (To find the total area $[ABC] = 48$, drop the altitudes from $A$ to $BC$, and call the foot of the altitude $F$. By the 6-8-10 triangle, the height $AF$ is $8$ and the area of $ABC$ is $48$.)

The second way of finding the area of triangle $ACD$ is $\frac{1}{2}bh$. The base is $AC=10$, and $DG$ is the height. Therefore,

\begin{align*} [ACD] = \frac{24x}{5} &= \frac{1}{2} \cdot 10 \cdot DG \\[5pt] \frac{24x}{25} &= DG \end{align*}

Now we have $DG$ in terms of $x$, we use the similar triangles $GCD$ and $FAC$ and set up the proportion \begin{align*} \frac{DG}{CF} &= \frac{GC}{FA} \\[5pt] \frac{\frac{24x}{25}}{6} &= \frac{5+\frac{x}{2}}{8} \\[5pt] x &= \frac{250}{39}. \end{align*} So, our answer is $\boxed{289}$. -epiconan

Solution 8 (Easiest way- Coordinates without bash)

Let $B=(0, 0)$, and $C=(12, 0)$. From there, we know that $A=(6, 8)$, so line $AB$ is $y=\frac{4}{3}x$. Hence, $D=(a, \frac{4}{3}a)$ for some $a$, and $BD=\frac{5}{3}a$ so $AD=10-\frac{5}{3}a$. Now, notice that by symmetry, $E=(6+a, 8-\frac{4}{3}a)$, so $ED^2=6^2+(8-\frac{8}{3}a)^2$. Because $AD=ED$, we now have $(10-\frac{5}{3})^2=6^2+(8-\frac{8}{3}a)^2$, which simplifies to $\frac{64}{9}a^2-\frac{128}{3}a+100=\frac{25}{9}a^2-\frac{100}{3}a+100$, so $\frac{39}{9}a=\frac{13}{3}a=\frac{28}{3}$, and $a=\frac{28}{13}$. It follows that $AD=10-\frac{5}{3}\times\frac{28}{13}=10-\frac{140}{39}=\frac{390-140}{39}=\frac{250}{39}$, and our answer is $250+39=\boxed{289}$.

-Stormersyle

Solution 9 one second accurate solve(1 variable equation)

Doing law of cosines we know that $\cos A$ is $\frac{7}{25}.$* Dropping the perpendicular from $D$ to $AE$ we get that

$$\frac{10-x}{2}=\frac{7x}{25}.$$ Solving for $x$ we get $\frac{250}{39}$ so our answer is $289$.

-harsha12345

• It is good to remember that doubling the smallest angle of a 3-4-5 triangle gives the larger (not right) angle in a 7-24-25 triangle.

Solution 10 (Law of Sines)

Let's label $\angle A = \theta$ and $\angle ECD = \alpha$. Using isosceles triangle properties and the triangle angle sum equation, we get

$$180-(180-2\theta+\alpha) + \frac{180-\theta}{2} + \left(\frac{180-\theta}{2} - \alpha\right) = 180.$$ Solving, we find $\theta = 2 \alpha$.

Relabelling our triangle, we get $\angle ABC = 90 - \alpha$. Dropping an altitude from $A$ to $BC$ and using the Pythagorean theorem, we find $[ABC] = 48$. Using the sine area formula, we see $\frac12 \cdot 10 \cdot 12 \cdot \sin(90-\alpha) = 48$. Plugging in our sine angle cofunction identity, $\sin(90-\alpha) = \cos(\alpha)$, we get $\alpha = \cos{^{-1}}{\frac45}$.

Now, using the Law of Sines on $\triangle ADE$, we get

$$\frac{\sin{2\alpha}}{\frac{p}{q}} = \frac{\sin{(180-4\alpha)}}{10-\frac{p}{q}}.$$ After applying numerous trigonometric and algebraic tricks, identities, and simplifications, such as $\sin{(180-4\alpha)}=\sin{4\alpha}$ and $\sin{\left(\cos{^{-1}}{\frac45}\right)} = \frac35$, we find $\frac{p}{q} = \frac{10\sin{2\alpha}}{\sin{4\alpha}+\sin{2\alpha}} = \frac{250}{39}$.

Therefore, our answer is $250 + 39 = \boxed{289}$.

~Tiblis

Solution 11 (Trigonometry)

We start by labelling a few angles (all of them in degrees). Let $\angle{BAC}=2\alpha = \angle{AED}, \angle{EDC}=\angle{ECD}=\alpha, \angle{DEC}=180-2\alpha, \angle{BDC}=3\alpha, \angle{DCB}=90-2\alpha, \angle{DBC}=90-\alpha$. Also let $AD=a$. By sine rule in $\triangle{ADE},$ we get $\frac{a}{\sin{2\alpha}}=\frac{10-a}{\sin{4\alpha}} \implies \cos{2\alpha}=\frac{5}{a}-\frac{1}{2}$ Using sine rule in $\triangle{ABC}$, we get $\sin{\alpha}=\frac{3}{5}$. Hence we get $\cos{2\alpha}=1-2\sin^2{\alpha}=1-\frac{18}{25}=\frac{7}{25}$. Hence $\frac{5}{a}=\frac{1}{2}+\frac{7}{25}=\frac{39}{50} \implies a=\frac{250}{39}$. Therefore, our answer is $\boxed{289}$

Alternatively, use sine rule in $\triangle{BDC}$. (It’s easier)

~Prabh1512

Solution 12 (Double Angle Identity)

We let $AD=x$. Then, angle $A$ is $2\sin^{-1}(\frac{3}{5})$ and so is angle $AED$. We note that $AE=10-x$. We drop an altitude from $D$ to $AE$, and we call the foot $F$. We note that $AF=\frac{10-x}{2}$. Using the double angle identity, we have $\sin(2\sin^{-1}(\frac{3}{5}))=2(\frac{3}{5})(\frac{4}{5})=\frac{24}{25}.$ Therefore, $DG=\frac{24}{25}AD.$ We now use the Pythagorean Theorem, which gives $(\frac{10-x}{2})^2+(\frac{24}{25}x)^2=x^2$. Rearranging and simplifying, this becomes $429x^2-12500x+62500=0$. Using the quadratic formula, this is $\frac{12500\pm\sqrt{12500^2-250000\cdot429}}{858}$. We take out a $10000$ from the square root and make it a $100$ outside of the square root to make it simpler. We end up with $\frac{12500\pm7000}{858}$. We note that this must be less than 10 to ensure that $10-x$ is positive. Therefore, we take the minus, and we get $\frac{5500}{858}=\frac{250}{39} \implies \fbox{289}.$

~john0512

Video Solution

https://www.youtube.com/watch?v=iE8paW_ICxw

https://youtu.be/dI6uZ67Ae2s ~yofro