Triplication

Geometry Level 5

In A B C \triangle ABC , A C = 4 AC = 4 , B C = 5 BC = 5 , and A B = 6 AB = 6 , point E E is on A B \overline{AB} so that A E AE = 1 2 E B \dfrac{1}{2}EB .

If the sums of C E CE and the measure of the segment from E E to the midpoint of C B \overline{CB} can be written as: 1 2 P + Q R , \frac{1}{2}\sqrt{P+Q\sqrt{R}}, where P , Q , R P,Q,R are all integers. Find P P .


The answer is 73.

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3 solutions

Guilherme Niedu
Mar 27, 2017

Let us define as F F the midpoint of B C \overline{BC} . A E = 2 \overline{AE} = 2 , B E = 4 \overline{BE} = 4 , B F = C F = 2.5 \overline{BF} = \overline{CF} = 2.5 . By cosine law:

B C 2 = A C 2 + A B 2 2 A C A B c o s ( A ^ ) \large \displaystyle \overline{BC}^2 = \overline{AC}^2 + \overline{AB}^2 -2 \cdot \overline{AC} \cdot \overline{AB}\ cos(\hat{A})

25 = 16 + 36 48 c o s ( A ^ ) \large \displaystyle 25 = 16 + 36 - 48 cos(\hat{A})

c o s ( A ^ ) = 9 16 \color{#20A900} \boxed{\large \displaystyle cos(\hat{A}) = \frac{9}{16} }

A C 2 = A B 2 + B C 2 2 A B B C c o s ( B ^ ) \large \displaystyle \overline{AC}^2 = \overline{AB}^2 + \overline{BC}^2 -2 \cdot \overline{AB}\cdot \overline{BC}\ cos(\hat{B})

16 = 25 + 36 60 c o s ( B ^ ) \large \displaystyle 16 = 25 + 36 - 60 cos(\hat{B})

c o s ( B ^ ) = 3 4 \color{#20A900} \boxed{\large \displaystyle cos(\hat{B}) = \frac{3}{4} }

C E 2 = A C 2 + A E 2 2 A C A E c o s ( A ^ ) \large \displaystyle \overline{CE}^2 = \overline{AC}^2 + \overline{AE}^2 -2 \cdot \overline{AC}\cdot \overline{AE}\ cos(\hat{A})

C E 2 = 16 + 4 9 \large \displaystyle \overline{CE}^2 = 16 + 4 - 9

C E = 11 \color{#20A900} \boxed{\large \displaystyle \overline{CE} = \sqrt{11} }

E F 2 = B E 2 + B F 2 2 B E B F c o s ( B ^ ) \large \displaystyle \overline{EF}^2 = \overline{BE}^2 + \overline{BF}^2 -2 \cdot \overline{BE}\cdot \overline{BF}\ cos(\hat{B})

E F 2 = 25 4 + 16 15 \large \displaystyle \overline{EF}^2 = \frac{25}{4} + 16 - 15

E F = 29 2 \color{#20A900} \boxed{\large \displaystyle \overline{EF} = \frac{\sqrt{29}}{2} }

C E + E F = 11 + 29 2 \large \displaystyle \overline{CE} + \overline{EF} = \sqrt{11} + \frac{\sqrt{29}}{2}

C E + E F = ( 11 + 29 2 ) 2 \large \displaystyle \overline{CE} + \overline{EF} = \sqrt{ \left ( \sqrt{11} + \frac{\sqrt{29}}{2} \right )^2 }

C E + E F = 11 + 29 4 + 319 \large \displaystyle \overline{CE} + \overline{EF} = \sqrt{ 11 + \frac{29}{4} + \sqrt{319} }

C E + E F = 1 2 73 + 4 319 \large \displaystyle \overline{CE} + \overline{EF} = \frac12 \sqrt{ 73 + 4\sqrt{319} }

A = 73 , B = 4 , C = 319 \color{#3D99F6} \boxed{\large \displaystyle A = 73} , \boxed{\large \displaystyle B = 4}, \boxed{\large \displaystyle C = 319}

Basically same as Mr. Guilherme Niedu and MR. Marta Reece but little different.
A E = 2 , a n d E B = 4 , l e t C A B = C A E = α , a n d M m d p o i n t o f C B . U s i n g C o s R u l e i n Δ s , C A B a n d C A E , C o s α = 4 2 + 6 2 5 2 2 4 6 = 9. A l s o C o s α = 4 2 + 2 2 C E 2 2 4 2 = 20 C E 2 . C E = 11 . I n Δ C E B , m e d i a n E M = 1 2 1 2 ( C E 2 + 6 2 5 2 ) = 1 2 29 . C E + E M = 1 2 44 + 1 2 29 = 1 2 44 + 29 + Q R P = 73. AE=2,~and~EB=4,~let~\angle CAB=\angle CAE=\alpha,~and~M~mdpoint~of~CB.\\ Using ~Cos~Rule~ in~\Delta s,~CAB~and~CAE,\\ Cos\alpha=\dfrac{4^2+6^2-5^2}{2*4*6}=9.\\ Also~ Cos\alpha=\dfrac{4^2+2^2-CE^2}{2*4*2}=20 - CE^2.\\ \therefore~CE=\sqrt{11}.\\ In ~\Delta~CEB,~median~EM=\frac 1 2 *\sqrt{\frac 1 2 *(CE^2+6^2 - 5^2)}=\frac 1 2\sqrt{29}.\\ \therefore~CE+EM=\frac 1 2 \sqrt{44}+\frac 1 2\sqrt{29}=\frac 1 2 \sqrt{44+29+Q\sqrt R}\\ \implies~P=\Large \color{#D61F06}{73} . .

Marta Reece
Mar 27, 2017

From A B C \triangle ABC we get c o s ( C A B ) = 4 2 + 6 2 5 2 2 × 4 × 6 = 9 16 cos(CAB)=\frac{4^2+6^2-5^2}{2\times4\times6}=\frac{9}{16}

From A E C \triangle AEC and again the law of cosines x = 4 2 + 2 2 2 × 2 × 4 × c o s ( C A B ) = 11 x=\sqrt{4^2+2^2-2\times2\times4\times cos(CAB)}=\sqrt{11}

Again from A B C \triangle ABC comes c o s ( A B C ) = 5 2 + 6 2 4 2 2 × 5 × 6 = 3 4 cos(ABC)=\frac{5^2+6^2-4^2}{2\times5\times6}=\frac{3}{4}

And from D E B \triangle DEB we calculate y = 2. 5 2 + 4 2 2 × 2.5 × 4 × c o s ( A B C ) = 29 2 y=\sqrt{2.5^2+4^2-2\times2.5\times4\times cos(ABC)}=\frac{\sqrt{29}}{2}

So x + y = 11 + 29 2 x+y=\sqrt{11}+\frac{\sqrt{29}}{2} .

This is a perfectly good answer. It does not, however, fit the required format. To do that, we have to show what it is a squire root of, in other words square it and put that under a square root. With a little simplification, that becomes.

x + y = 1 2 73 + 4 319 x+y=\frac{1}{2}\sqrt{73+4\sqrt{319}} .

We can directly apply Stewart's theorem.

Harry Jones - 4 years, 2 months ago

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and appolonius theorem which is the special case of it.

Ayush G Rai - 4 years, 2 months ago

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