Difference between revisions of "2014 AMC 12B Problems/Problem 24"
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We desire <math>3c+a+b = 3\cdot 12 + \frac{44}{3} + \frac{27}{2} = \frac{216+88+81}{6}=\frac{385}{6}</math>, so it follows that the answer is <math>385 + 6 = \fbox{\textbf{(D) }391}</math> | We desire <math>3c+a+b = 3\cdot 12 + \frac{44}{3} + \frac{27}{2} = \frac{216+88+81}{6}=\frac{385}{6}</math>, so it follows that the answer is <math>385 + 6 = \fbox{\textbf{(D) }391}</math> | ||
+ | |||
+ | ==Solution 3 (Ptolemy's but Quicker)== | ||
+ | |||
+ | Let us set <math>x</math> to be <math>AC=BD=CE</math> and <math>y</math> to be <math>BE</math> and <math>z</math> to be <math>AD</math>. It follow from applying [[Ptolemy's Theorem]] on <math>ABCD</math> to get <math>x^2=9+10z</math>. Applying Ptolemy's on <math>ACDE</math> gives <math>xz=42+10x</math>; and applying Ptolemy's on <math>BCDE</math> gives <math>x^2=100+3y</math>. So, we have the have the following system of equations: | ||
+ | |||
+ | <cmath> | ||
+ | \begin{align} | ||
+ | x^2 &= 9+10z \ | ||
+ | x^2 &= 100+3y \ | ||
+ | xz &= 42+10x | ||
+ | \end{align} | ||
+ | </cmath> | ||
+ | |||
+ | From <math>(3)</math>, we have <math>42=(z-10)x</math>. Isolating the x gives <math>x=\dfrac{42}{z-10}</math>. By setting <math>(1)</math> and <math>(2)</math> equal, we have <math>x^2=9+10z=100+3y</math>. Manipulating it gives <math>3y=10z-91</math>. Finally, plugging back into <math>(2)</math> gives <math>x^2=100+10z-91=10z+9</math>. Plugging in the <math>x=\dfrac{42}{z-10}</math> as well gives | ||
+ | |||
+ | <cmath> | ||
+ | \begin{align*} | ||
+ | \left(\frac{42}{z-10}\right)^2 &= 10z+9\ | ||
+ | 10z^3 - 191z^2 + 820z + 900 &= 1764\ | ||
+ | 10z^3 - 191z^2 + 820z - 864 &= 0\ | ||
+ | (5z-8)(2z-27)(z-4) &=0 | ||
+ | \end{align*} | ||
+ | </cmath> | ||
+ | |||
+ | It is impossible for <math>z<10</math> for <math>x<0</math>; that means <math>z=\frac{27}{2}</math>. That means <math>x = 12</math> and <math>y = \frac{44}{3}</math>. | ||
+ | |||
+ | Thus, the sum of all diagonals is <math>3x+y+z = 3\cdot 12 + \frac{44}{3} + \frac{27}{2} = 385/6</math>, which implies our answer is <math>m+n = 385+6 = \fbox{391 \textbf{(D)}}</math>. | ||
+ | |||
+ | ~ sml1809 | ||
== See also == | == See also == |
Revision as of 14:21, 3 August 2022
Contents
[hide]Problem
Let be a pentagon inscribed in a circle such that
,
, and
. The sum of the lengths of all diagonals of
is equal to
, where
and
are relatively prime positive integers. What is
?
Video Solution by Punxsutawney Phil
https://www.youtube.com/watch?v=1-2vT_GIceA
Solution 1
Let ,
, and
. Let
be on
such that
.
In
we have
. We use the Law of Cosines on
to get
. Eliminating
we get
which factorizes as
Discarding the negative roots we have
. Thus
. For
, we use Ptolemy's theorem on cyclic quadrilateral
to get
. For
, we use Ptolemy's theorem on cyclic quadrilateral
to get
.
The sum of the lengths of the diagonals is so the answer is
Solution 2
Let denote the length of a diagonal opposite adjacent sides of length
and
,
for sides
and
, and
for sides
and
. Using Ptolemy's Theorem on the five possible quadrilaterals in the configuration, we obtain:
Using equations and
, we obtain:
and
Plugging into equation , we find that:
Or similarly into equation to check:
, being a length, must be positive, implying that
. In fact, this is reasonable, since
in the pentagon with apparently obtuse angles. Plugging this back into equations
and
we find that
and
.
We desire , so it follows that the answer is
Solution 3 (Ptolemy's but Quicker)
Let us set to be
and
to be
and
to be
. It follow from applying Ptolemy's Theorem on
to get
. Applying Ptolemy's on
gives
; and applying Ptolemy's on
gives
. So, we have the have the following system of equations:
From , we have
. Isolating the x gives
. By setting
and
equal, we have
. Manipulating it gives
. Finally, plugging back into
gives
. Plugging in the
as well gives
It is impossible for for
; that means
. That means
and
.
Thus, the sum of all diagonals is , which implies our answer is
.
~ sml1809
See also
2014 AMC 12B (Problems • Answer Key • Resources) | |
Preceded by Problem 23 |
Followed by Problem 25 |
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 | |
All AMC 12 Problems and Solutions |
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