Extended Bisector

Geometry Level 3

Triangle A B C ABC is inscribed in a unit circle. The three bisectors of the angle A A , B B and C C are extended to intersect the circle at A A' , B B' and C C' respectively. Determine the value of

A A cos A 2 + B B cos B 2 + C C cos C 2 sin A + sin B + sin C \frac{AA'\cos\frac{A}{2}+BB'\cos\frac{B}{2}+CC'\cos\frac{C}{2}}{\sin A+\sin B+\sin C}


The answer is 2.

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4 solutions

James Pohadi
Apr 26, 2017

By observing triangle A B A ABA' , we have A A sin ( B + 1 2 A ) = 2 r = 2 × 1 = 2 A A = 2 sin ( B + 1 2 A ) \dfrac{AA'}{\sin( B + \frac{1}{2}A)} = 2r =2 \times 1 = 2 \implies AA'=2\sin( B + \frac{1}{2}A)

The same process for triangle B C B BCB' and C A C CAC' , we have B B = 2 sin ( C + 1 2 B ) BB'=2 \sin( C + \frac{1}{2}B) and C C = 2 sin ( A + 1 2 C ) CC'=2 \sin( A + \frac{1}{2}C)

Then, A A cos A 2 + B B cos B 2 + C C cos C 2 sin A + sin B + sin C \dfrac{AA'\cos\frac{A}{2}+BB'\cos\frac{B}{2}+CC'\cos\frac{C}{2}}{\sin A+\sin B+\sin C} is

= 2 sin ( B + 1 2 A ) cos A 2 + 2 sin ( C + 1 2 B ) cos B 2 + 2 sin ( A + 1 2 C ) cos C 2 sin A + sin B + sin C = sin ( B + A ) + sin B + sin ( C + B ) + sin C + sin ( A + C ) + s i n A sin A + sin B + sin C 2 sin A cos B = sin ( A + B ) + sin ( A B ) = sin ( 180 C ) + sin ( 180 A ) + sin ( 180 B ) + sin A + sin B + sin C sin A + sin B + sin C A + B + C = 180 = 2 ( sin A + sin B + sin C ) sin A + sin B + sin C = 2 \begin{aligned} & =\dfrac{2\sin( B + \frac{1}{2}A)\cos\frac{A}{2}+2\sin( C + \frac{1}{2}B)\cos\frac{B}{2}+2\sin( A + \frac{1}{2}C)\cos\frac{C}{2}}{\sin A+\sin B+\sin C} \\ & =\dfrac{\sin( B + A)+\sin B+\sin( C + B)+\sin C+\sin( A + C)+sinA}{\sin A+\sin B+\sin C} &\small \color{#3D99F6}{ 2 \sin A \cos B = \sin (A+B) + \sin (A-B)} \\ & =\dfrac{\sin(180-C)+\sin(180-A)+\sin(180-B)+\sin A+\sin B+\sin C}{\sin A+\sin B+\sin C} &\small \color{#3D99F6}{ A+B+C=180 }\\ & =\dfrac{2(\sin A+\sin B+\sin C)}{\sin A+\sin B+\sin C} \\ & =\boxed{2} \end{aligned}

Observe that A B = A C = a 2 cos A 2 A'B=A'C=\dfrac{a}{2\cos\tfrac{A}{2}} . By Ptolemy's Theorem, A B A C + A C A B = A A B C AB\cdot A'C+AC\cdot A'B=AA'\cdot BC . Substitution and rearrangement yield A A cos A 2 = b + c 2 AA'\cos\tfrac{A}{2}=\tfrac{b+c}{2} and similarly for B B BB' and C C CC' . Therefore, the desired sum is b + c 2 + c + a 2 + a + b 2 sin A + sin B + sin C = a + b + c sin A + sin B + sin C \dfrac{\tfrac{b+c}{2}+\tfrac{c+a}{2}+\tfrac{a+b}{2}}{\sin A+\sin B+\sin C}=\dfrac{a+b+c}{\sin A+\sin B+\sin C} . By the Extended Law of Sines, a = 2 sin A a=2\sin A and similarly for b b and c c , so the answer is 2 \boxed{2} .

Aaaaa Bbbbb
May 15, 2014

With equilateral triangle, denote R is radius of circumscribed circle. it has: A A = B B = C C = 2 R , s i n A = s i n B = s i n C = c o s ( A 2 ) = c o s ( B 2 ) = c o s ( C 2 ) = 3 2 AA'=BB'=CC'=2R, sin A=sin B=sin C=cos(\frac{A}{2})=cos(\frac{B}{2})=cos(\frac{C}{2})=\frac{\sqrt{3}}{2} R e s u l t = 3 3 3 3 2 = 2 \Rightarrow Result=\frac{3\sqrt{3}}{\frac{3\sqrt{3}}{2}}=\boxed{2}

Vishal Sharma
May 9, 2014

Well, a short way to do it can be to assume the triangle to be equilateral... The expression then reduces to 2 R 2R Where R R is the radius of the circumcircle , which here is "1" Hence answer = 2

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