Quite the same

Geometry Level 2

Triangle A B C ABC is inscribed in equilateral triangle P Q R PQR . If P C = 3 , B P = C Q = 2 PC = 3, BP = CQ = 2 , and angle A C B = 9 0 ACB = 90^\circ , then compute A Q AQ .

7 3 \frac7 3 9 7 \frac9 7 4 3 \frac4 3 3 2 \frac3 2 8 5 \frac 8 5

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2 solutions

Chew-Seong Cheong
Apr 13, 2015

Since P Q R \triangle PQR is an equilateral triangle, we have R Q = P Q = P C + C Q = 3 + 2 = 5 RQ=PQ=PC+CQ=3+2=5 . Let A R = x AR=x . By cosine rule, we have:

{ A B 2 = B R 2 + A R 2 2 B R ˙ A R cos 6 0 = 3 2 + x 2 3 x = x 2 3 x + 9 B C 2 = B P 2 + P C 2 2 B P ˙ P C cos 6 0 = 2 2 + 3 2 2 ( 3 ) = 7 A C 2 = A Q 2 + C Q 2 2 A Q ˙ C Q cos 6 0 = ( 5 x ) 2 + 2 2 2 ( 5 x ) = x 2 8 x + 19 \begin{cases} AB^2 = BR^2+AR^2-2BR\dot{}AR \cos 60^\circ = 3^2 +x^2 - 3x & = x^2 - 3x + 9 \\ BC^2 = BP^2+PC^2-2BP\dot{}PC \cos 60^\circ = 2^2+3^2-2(3) & = 7 \\ AC^2 = AQ^2 + CQ^2 -2AQ\dot{}CQ \cos 60^\circ = (5-x)^2+2^2-2(5-x) & = x^2 - 8x + 19 \end{cases}

By Pythagorean theorem, we have B C 2 + A C 2 = A B 2 BC^2 +AC^2 = AB^2 :

x 2 3 x + 9 = 7 + x 2 8 x + 19 5 x = 17 x = 17 5 A Q = 5 x = 5 17 5 = 8 5 \begin{aligned} x^2 - 3x + 9 & = 7 + x^2 - 8x + 19 \\ \Rightarrow 5x & = 17 \\ x & = \frac{17}{5} \\ \Rightarrow AQ & = 5 - x = 5 - \frac{17}{5} = \boxed{\dfrac{8}{5}} \end{aligned}

Moderator note:

There's a much simpler approach.

Because P Q R \triangle PQR is an equilateral triangle, R = P = Q = 6 0 \angle R = \angle P = \angle Q = 60 ^\circ . Labeling x = A R x = AR , so A Q = 5 x AQ = 5-x . Then by Cosine Rule , we can find the length of B C , A B , A C BC,AB,AC . By Pythagorean Theorem , B C 2 + A C 2 = A B 2 BC^2 + AC^2 = AB^2 . Can you carry on from here?

Nguyen Thanh Long
Apr 13, 2015

B C = 2 2 + 3 2 2 3 2 ( 1 / 2 ) BC=\sqrt{2^2+3^2-2*3*2*(1/2)} P B s i n ( P C B ) = B C s i n ( B P C ) \frac{PB}{sin(\angle{PCB})}=\frac{BC}{sin(\angle{BPC})} B C P = a s i n ( 0.654 ) A C Q = 90 B C P \angle{BCP}=asin(0.654) \rightarrow \angle{ACQ}=90-\angle{BCP} C A Q = 120 A C Q \rightarrow \angle{CAQ}=120-\angle{ACQ} A Q s i n ( A C Q ) = C Q s i n ( C A Q ) \frac{AQ}{sin(\angle{ACQ})}=\frac{CQ}{sin(\angle{CAQ})} A Q = 8 5 AQ=\boxed{\frac{8}{5}}

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