AIME 2011 I · 第 4 题

Solution 1

Extend ${CM}$ and ${CN}$ such that they intersect line ${AB}$ at points $P$ and $Q$, respectively.

Since ${BM}$ is the angle bisector of angle $B$ and ${CM}$ is perpendicular to ${BM}$, $\triangle BCP$ must be an isoceles triangle, so $BP=BC=120$, and $M$ is the midpoint of ${CP}$. For the same reason, $AQ=AC=117$, and $N$ is the midpoint of ${CQ}$. Hence $MN=\tfrac 12 PQ$. Since

$$PQ=BP+AQ-AB=120+117-125=112,$$ so $MN=\boxed{056}$.

Solution 2

Let $I$ be the incenter of $ABC$. Since $I$ lies on $BM$ and $AN$, $IM \perp MC$ and $IN \perp NC$, so $\angle IMC + \angle INC = 180^\circ$. This means that $CMIN$ is a cyclic quadrilateral. By the Law of Sines, $\frac{MN}{\sin \angle MIN} = \frac{2R}{\sin \angle CMI} = 2R = CI$, where $R$ is the radius of the circumcircle of $CMIN$. Since $\sin \angle MIN = \sin \angle BIA = \sin (90^\circ + \tfrac 12 \angle BCA) = \cos \tfrac 12 \angle BCA = \cos \angle BCI$, we have that $MN = CI \cdot \sin \angle MIN = CI \cdot \cos \angle BCI$. Letting $H$ be the point of contact of the incircle of $ABC$ with side $BC$, we have $MN = CI \cdot \cos \angle BCI = CI \cdot \frac{CH}{CI} = CH$. Thus, $MN = s - AB = \frac{117+120-125}{2}=\boxed{056}$.

Solution 3 (Bash)

Project $I$ onto $AC$ and $BC$ as $D$ and $E$. $ID$ and $IE$ are both in-radii of $\triangle ABC$ so we get right triangles with legs $r$ (the in-radius length) and $s - c = 56$. Since $IC$ is the hypotenuse for the 4 triangles ($\triangle INC, \triangle IMC, \triangle IDC,$ and $\triangle IEC$), $C, D, M, I, N, E$ are con-cyclic on a circle we shall denote as $\omega$ which is also the circumcircle of $\triangle CMN$ and $\triangle CDE$. To find $MN$, we can use the Law of Cosines on $\angle MON \implies MN^2 = 2R^2(1 - \cos{2\angle MCN})$ where $O$ is the center of $\omega$. Now, the circumradius $R$ can be found with Pythagorean Theorem with $\triangle CDI$ or $\triangle CEI$: $r^2 + 56^2 = (2R)^2$. To find $r$, we can use the formula $rs = [ABC]$ and by Heron's, $[ABC] = \sqrt{181 \cdot 61 \cdot 56 \cdot 64} \implies r = \sqrt{\frac{61 \cdot 56 \cdot 64}{181}} \implies 2R^2 = \frac{393120}{181}$. To find $\angle MCN$, we can find $\angle MIN$ since $\angle MCN = 180 - \angle MIN$. $\angle MIN = \angle MIC + \angle NIC = 180 - \angle BIC + 180 - \angle AIC = 180 - (180 - \frac{\angle A + \angle C}{2}) + 180 - (180 - \frac{\angle B + \angle C}{2}) = \frac{\angle A + \angle B + \angle C}{2} + \frac{\angle C}{2}$. Thus, $\angle MCN = 180 - \frac{\angle A + \angle B + \angle C}{2} - \frac{\angle C}{2}$ and since $\angle A + \angle B + \angle C = 180$, we have $\angle A + \angle B + \angle C - \frac{\angle A + \angle B + \angle C}{2} - \frac{\angle C}{2} = \frac{\angle A + \angle B}{2}$. Plugging this into our Law of Cosines (LoC) formula gives $MN^2 = 2R^2(1 - \cos{\angle A + \angle B}) = 2R^2(1 + \cos{\angle C})$. To find $\cos{\angle C}$, we use LoC on $\triangle ABC \implies \cos{\angle C} = \frac{120^2 + 117^2 - 125^2}{2 \cdot 117 \cdot 120} = \frac{41 \cdot 19}{117 \cdot 15}$. Our formula now becomes $MN^2 = \frac{393120}{181} + \frac{2534}{15 \cdot 117}$. After simplifying, we get $MN^2 = 3136 \implies MN = \boxed{056}$.

--lucasxia01

Solution 4

Because $\angle CMI = \angle CNI = 90$, $CMIN$ is cyclic.

Applying Ptolemy's theorem on CMIN:

$CN \cdot MI+CM \cdot IN=CI \cdot MN$ $CI^2(\cos \angle ICN \sin \angle ICM + \cos \angle ICM \sin \angle ICN) = CI \cdot MN$

$MN = CI \sin \angle MCN$ by sine angle addition formula.

$\angle MCN = 180 - \angle MIN = 90 - \angle BCI$.

Let $H$ be where the incircle touches $BC$, then $CI \cos \angle BCI = CH = \frac{a+b-c}{2}$. $a=120, b=117, c=125$, for a final answer of $\boxed{056}$.

Note: This is similar to Solution 2 after the first four lines

Solution 5 (Trig Bash)

Applying Ptolemy's Theorem on the cyclic quadrilateral $MINC$, we find that

$MI\cdot CN + IN\cdot MC = MN\cdot IC$.

$\angle CIN=\frac{\alpha+\gamma}{2}$ and $\angle MIC=\frac{\beta+\gamma}{2}$ by the Exterior Angle Theorem, so from properties of sine and cosine, we can find that

$MI=IC\cdot\cos\left(\frac{\beta+\gamma}{2}\right),$ $MC=IC\cdot\sin\left(\frac{\beta+\gamma}{2}\right),$ $IN=IC\cdot\cos\left(\frac{\alpha+\gamma}{2}\right),$ $CN=IC\cdot\sin\left(\frac{\alpha+\gamma}{2}\right).$

Plugging in the values and simplifying results in $MN = IC\cdot\sin\left(\frac{\alpha+\beta+2\gamma}{2}\right)$ by the angle-addition identity $\sin(A+B)=\sin(A)\cos(B)+\sin(B)\cos(A)$.

Before we continue, we would like to simplify the value in the sine function. We see that $\frac{\alpha+\beta+2\gamma}{2}=\frac{\gamma}{2}+\frac{\alpha+\beta+\gamma}{2}=\frac{\gamma}{2}+90$. Using the fact that $\cos(A)=\sin(90-A)$ results in

$\sin\left(\frac{\alpha+\beta+2\gamma}{2}\right)=\sin\left(90+\frac{\gamma}{2}\right)=\sin\left(90-(-\frac{\gamma}{2})\right)=\cos\left(-\frac{\gamma}{2}\right)=\cos\left(\frac{\gamma}{2}\right).$

How do we simplify $IC$? Well, we can perform the Law of Sines on triangle $AIC$. This results in:

$\frac{AC}{\sin(\angle AIC)}=\frac{IC}{\sin\left(\frac{\alpha}{2}\right)}$

The value of $\angle AIC$ is $\frac{\alpha+2\beta+\gamma}{2}$ by the Exterior Angle Theorem on $\triangle ABI$, so the value of $\sin(\angle AIC)$ is equivalent to the value of $\cos\left(\frac{\beta}{2}\right)$ by a similar argument as above. Then rearranging yields $IC = b\cdot\frac{\sin\left(\frac{\alpha}{2}\right)}{\cos\left(\frac{\beta}{2}\right)}$.

Going back to the previous formula $MN = IC\cdot\sin\left(\frac{\alpha+\beta+2\gamma}{2}\right)$ and substituting values yields:

$MN = b\cdot\frac{\sin\left(\frac{\alpha}{2}\right)\cos\left(\frac{\gamma}{2}\right)}{\cos\left(\frac{\beta}{2}\right)}$.

Finally, using the formulae $\sin\left(\frac{\alpha}{2}\right)=\sqrt{\frac{(s-b)(s-c)}{bc}}$ and $\cos\left(\frac{\alpha}{2}\right)=\sqrt{\frac{s(s-a)}{bc}}$ (where $s$ is half the perimeter of the triangle), we reach our final value:

$MN = b\cdot\frac{\sqrt{\frac{(s-b)(s-c)}{bc}}\cdot\sqrt{\frac{s(s-c)}{ab}}}{\sqrt{\frac{s(s-b)}{ac}}}$ $=\frac{\sqrt{s(s-b)(s-c)^2}}{\sqrt{s(s-b)}}$ $=\sqrt{(s-c)^2}$ $=s-c$ $=181-125$ $=\boxed{056}.$

Video Solution

https://www.youtube.com/watch?v=yIUBhWiJ4Dk ~Mathematical Dexterity

Video Solution

https://www.youtube.com/watch?v=vkniYGN45F4

~Shreyas S

Alternate Solution: https://www.youtube.com/watch?v=L2OzYI0OJsc&t=12s