1971 AHSME Problems/Problem 31
Contents
Problem
Quadrilateral is inscribed in a circle with side
, a diameter of length
. If sides
and
each have length
,
then side
has length
Solution 1
Note that the length 4 forms a semicircle. We can then use the Law of Cosines. Take the center and form a line segment with the other two points.
Let's find the cosine of angle . The cosine of that angle is 7/8. We use the double cosine angle to find angle
:
Therefore,
Applying the Law of Cosines to the triangle , we get
(because
)
Therefore,
and we find that !
Solution 2
By the properties of a circle, we see that angle and angle
are both right angles. Thus, if we formulate the diagonal lengths of the cyclic quadrilateral using the Pythagorean theorem, we can finish the problem with Ptolemy's Theorem.
With that train of thought, we see that
To let us formulate that rest of our lengths, let . Then, similar to the above,
Now that we have all of the side lengths and diagonal lengths in one variable, we use Ptolemy's Theorem to finish from here:
Since the negative solution is extraneous, we see that our solution is
See Also
1971 AHSC (Problems • Answer Key • Resources) | ||
Preceded by Problem 30 |
Followed by Problem 32 | |
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