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Difference between revisions of "1993 AHSME Problems/Problem 23"

m (Solution)
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We get:
We get:
<math>AB /sin(\angle{AXB}) = AX / sin(\angle{ABX})</math>
<math>\frac{AB}{sin(\angle{AXB})} =\frac{AX}{sin(\angle{ABX})}</math>
That's equal to
That's equal to
<math>cos(6)/ sin(180-18) = AX / sin 12</math>
<math>\frac{cos(6)}{sin(180-18)} =\frac{AX}{sin(12)}</math>
Therefore, our answer is equal to:
Therefore, our answer is equal to:

Revision as of 16:05, 3 August 2019


[asy] draw(circle((0,0),10),black+linewidth(.75)); draw((-10,0)--(10,0),black+linewidth(.75)); draw((-10,0)--(9,sqrt(19)),black+linewidth(.75)); draw((-10,0)--(9,-sqrt(19)),black+linewidth(.75)); draw((2,0)--(9,sqrt(19)),black+linewidth(.75)); draw((2,0)--(9,-sqrt(19)),black+linewidth(.75)); MP("X",(2,0),N);MP("A",(-10,0),W);MP("D",(10,0),E);MP("B",(9,sqrt(19)),E);MP("C",(9,-sqrt(19)),E); [/asy]

Points $A,B,C$ and $D$ are on a circle of diameter $1$, and $X$ is on diameter $\overline{AD}.$

If $BX=CX$ and $3\angle{BAC}=\angle{BXC}=36^\circ$, then $AX=$

$\text{(A) } cos(6^\circ)cos(12^\circ)sec(18^\circ)\quad\\ \text{(B) } cos(6^\circ)sin(12^\circ)csc(18^\circ)\quad\\ \text{(C) } cos(6^\circ)sin(12^\circ)sec(18^\circ)\quad\\ \text{(D) } sin(6^\circ)sin(12^\circ)csc(18^\circ)\quad\\ \text{(E) } sin(6^\circ)sin(12^\circ)sec(18^\circ)$


We have all the angles we need, but most obviously, we see that right angle in triangle $ABD$.

Note also that angle $BAD$ is 6 degrees, so length $AB = cos(6)$ because the diameter, $AD$, is 1.

Now, we can concentrate on triangle $ABX$ (after all, now we can decipher all angles easily and use Law of Sines).

We get:

$\frac{AB}{sin(\angle{AXB})} =\frac{AX}{sin(\angle{ABX})}$

That's equal to

$\frac{cos(6)}{sin(180-18)} =\frac{AX}{sin(12)}$

Therefore, our answer is equal to: $\fbox{B}$

Note that $sin(162) = sin(18)$, and don't accidentally put $\fbox{C}$ because you thought 1/sin was sec!

See also

1993 AHSME (ProblemsAnswer KeyResources)
Preceded by
Problem 22
Followed by
Problem 24
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 26 27 28 29 30
All AHSME Problems and Solutions

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