AIME 1983 · 第 4 题

Solution 1

Because we are given a right angle, we look for ways to apply the Pythagorean Theorem. Let the foot of the perpendicular from $O$ to $AB$ be $D$ and let the foot of the perpendicular from $O$ to the line $BC$ be $E$. Let $OE=x$ and $OD=y$. We're trying to find $x^2+y^2$.

Applying the Pythagorean Theorem, $OA^2 = OD^2 + AD^2$ and $OC^2 = EC^2 + EO^2$.

Thus, $\left(\sqrt{50}\right)^2 = y^2 + (6-x)^2$, and $\left(\sqrt{50}\right)^2 = x^2 + (y+2)^2$. We solve this system to get $x = 1$ and $y = 5$, such that the answer is $1^2 + 5^2 = \boxed{026}$.

Solution 2 (Trig)

We'll use the law of cosines. Let $O$ be the center of the circle; we wish to find $OB$. We know how long $OA$ and $AB$ are, so if we can find $\cos \angle OAB$, we'll be in good shape.

We can find $\cos \angle OAB$ using angles $OAC$ and $BAC$. First we note that by Pythagoras,

$$AC = \sqrt{AB^2 + BC^2} = \sqrt{36 + 4} = \sqrt{40} = 2 \sqrt{10}.$$ If we let $M$ be the midpoint of $AC$, that mean that $AM = \sqrt{10}$. Since $\triangle OAC$ is isosceles ($OA = OC$ from the definition of a circle), $M$ is also the foot of the altitude from $O$ to $AC.$

It follows that $OM = \sqrt{40} = 2 \sqrt{10}$. Therefore

$$\begin{aligned} \cos \angle OAC = \frac{\sqrt{10}}{\sqrt{50}} &= \frac{1}{\sqrt{5}}, \\ \sin \angle OAC = \frac{2 \sqrt{10}}{\sqrt{50}} &= \frac{2}{\sqrt{5}}. \end{aligned}$$ Meanwhile, from right triangle $ABC,$ we have

$$\begin{aligned} \cos \angle BAC = \frac{6}{\sqrt{40}} &= \frac{3}{\sqrt{10}}, \\ \sin \angle BAC = \frac{2}{\sqrt{40}} &= \frac{1}{\sqrt{10}}. \end{aligned}$$ This means that by the angle subtraction formulas,

$$\begin{aligned} \cos \angle OAB &= \cos (\angle OAC - \angle BAC) \\ &= \cos \angle OAC \cos \angle BAC + \sin \angle OAC \sin \angle BAC \\ &= \frac{1}{\sqrt{5}} \cdot \frac{3}{\sqrt{10}} + \frac{2}{\sqrt{5}} \cdot \frac{1}{\sqrt{10}} \\ &= \frac{5}{5 \sqrt{2}} = \frac{1}{\sqrt{2}}. \end{aligned}$$ Now we have all we need to use the law of cosines on $\triangle OAB.$ This tells us that

$$\begin{aligned} OB^2 &= AO^2 + AB^2 - 2 AO \cdot AB \cdot \cos \angle OAB \\ &= 50 + 36 - 2 \cdot 5 \sqrt{2} \cdot 6 \cdot \frac{1}{\sqrt{2}} \\ &= 86 - 2 \cdot 5 \cdot 6 \\ &= 26. \end{aligned}$$

Solution 3

Mark the midpoint $M$ of $AC$. Then, drop perpendiculars from $O$ to $AB$ (with foot $T_1$), $M$ to $OT_1$ (with foot $T_2$), and $M$ to $AB$ (with foot $T_3$).

First notice that by computation, $OAC$ is a $\sqrt {50} - \sqrt {40} - \sqrt {50}$ isosceles triangle, so $AC = MO$. Then, notice that $\angle MOT_2 = \angle T_3MO = \angle BAC$. Therefore, the two blue triangles are congruent, from which we deduce $MT_2 = 2$ and $OT_2 = 6$. As $T_3B = 3$ and $MT_3 = 1$, we subtract and get $OT_1 = 5,T_1B = 1$. Then the Pythagorean Theorem tells us that $OB^2 = \boxed{026}$.

Solution 4 (Trig)

Draw segment $OB$ with length $x$, and draw radius $OQ$ such that $OQ$ bisects chord $AC$ at point $M$. This also means that $OQ$ is perpendicular to $AC$. By the Pythagorean Theorem, we get that $AC=\sqrt{(BC)^2+(AB)^2}=2\sqrt{10}$, and therefore $AM=\sqrt{10}$. Also by the Pythagorean theorem, we can find that $OM=\sqrt{50-10}=2\sqrt{10}$.

Next, find $\angle BAC=\arctan{\left(\frac{2}{6}\right)}$ and $\angle OAM=\arctan{\left(\frac{2\sqrt{10}}{\sqrt{10}}\right)}$. Since $\angle OAB=\angle OAM-\angle BAC$, we get

$$\angle OAB=\arctan{2}-\arctan{\frac{1}{3}}$$ $$\tan{(\angle OAB)}=\tan{(\arctan{2}-\arctan{\frac{1}{3}})}$$ By the subtraction formula for $\tan$, we get

$$\tan{(\angle OAB)}=\frac{2-\frac{1}{3}}{1+2\cdot \frac{1}{3}}$$ $$\tan{(\angle OAB)}=1$$ $$\cos{(\angle OAB)}=\frac{1}{\sqrt{2}}$$ Finally, by the Law of Cosines on $\triangle OAB$, we get

$$x^2=50+36-2(6)\sqrt{50}\frac{1}{\sqrt{2}}$$ $$x^2=\boxed{026}.$$

Solution 5

We use coordinates. Let the circle have center $(0,0)$ and radius $\sqrt{50}$; this circle has equation $x^2 + y^2 = 50$. Let the coordinates of $B$ be $(a,b)$. We want to find $a^2 + b^2$. $A$ and $C$ with coordinates $(a,b+6)$ and $(a+2,b)$, respectively, both lie on the circle. From this we obtain the system of equations

$a^2 + (b+6)^2 = 50$ $(a+2)^2 + b^2 = 50$

After expanding these terms, we notice by subtracting the first and second equations, we can cancel out $a^2$ and $b^2$. after substituting $a=3b+8$ and plugging back in, we realize that $(a,b)=(-7,-5)$ or $(5,-1)$. Since the first point is out of the circle, we find that $(5,-1)$ is the only relevant answer. This paragraph is written by ~hastapasta.

Solving, we get $a=5$ and $b=-1$, so the distance is $a^2 + b^2 = \boxed{026}$.

Solution 6 (Trigonometry)

I will use the law of cosines in triangle $\triangle OAC$ and $\triangle OBC$.

$AC = \sqrt{AB^2 + BC^2} = \sqrt{6^2 + 2^2} = 2 \sqrt{10}$ $\cos \angle ACB = \frac{2}{2\sqrt{10}} = \frac{1}{\sqrt{10}}$ $\cos \angle ACO = \frac{AC^2+OC^2-OA^2}{2 \cdot AC \cdot OC} = \frac{(2\sqrt{10})^2+(\sqrt{50})^2-(\sqrt{50})^2}{2 \cdot 2\sqrt{10} \cdot \sqrt{50}} = \frac{1}{\sqrt{5}}$ $\sin \angle ACB = \sqrt{1-\cos^2 \angle ACB} = \sqrt{1-(\frac{1}{\sqrt{10}})^2} = \frac{3}{\sqrt{10}}$ $\sin \angle ACO = \sqrt{1-\cos^2 \angle ACO} = \sqrt{1-(\frac{1}{\sqrt{5}})^2} = \frac{2}{\sqrt{5}}$ $\cos \angle OCB = \cos (\angle ACB - \angle ACO) = \cos \angle ACB \cdot \cos \angle ACO + \sin \angle ACB \cdot \sin \angle ACO = \frac{1}{\sqrt{10}} \cdot \frac{1}{\sqrt{5}} + \frac{3}{\sqrt{10}} \cdot \frac{2}{\sqrt{5}} = \frac{7}{5\sqrt{2}}$ $OB^2 = OC^2 + BC^2 - 2 \cdot OC \cdot BC \cdot \cos \angle OCB = (\sqrt{50})^2 + 2^2 - 2 \cdot \sqrt{50} \cdot 2 \cdot \frac{7}{5\sqrt{2}} = 50 + 4 - 28 = \boxed{026}$

~isabelchen

Solution 7

Notice that $50=5^2+5^2=7^2+1^2$, and by the size of the diagram, it seems reasonable that $OA$ represents $5^2+5^2$, and $OC$ means the $7^1+1^2$, and indeed, the values work ($7-5=2$ and $5+1=6$), so $OB^2=5^2+1^2=\boxed{026}$

Note: THIS IS NOT A RELIABLE SOLUTION, as diagrams on the tests are not usually drawn to scale.

Video Solution by Pi Academy

https://youtu.be/fHFD0TEfBnA?si=DRKVU_As7Rv0ou5D

~ Pi Academy