Central attention

Geometry Level 4

In a triangle ABC, with BC as diameter a circle is drawn such that it passes through centroid(G) of triangle ABC.If BG=8 units CG=15 units then what is the value of

A B 2 + A C 2 ? AB^2 + AC^2?


The answer is 1445.

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2 solutions

Sanjeet Raria
Sep 16, 2014

Let D be the mid point of BC, therefore the centre of the circle. Angles BGC is angle in semi circle so it's a right angle. Using Pythagoras, we have the the radius of the circle B D = D C = D G = 17 2 BD=DC=DG=\frac{17}{2} Now since the centroid divides the median in 2:1, so we can find out the median A D = 3 D G = 51 2 AD=3DG=\frac{51}{2} Now using the famous Apollonius theorem A B 2 + A C 2 = 2 ( A D 2 + B D 2 ) AB^2+AC^2=2(AD^2+BD^2) We have A B 2 + A C 2 = 2 ( ( 51 2 ) 2 + ( 17 2 ) 2 ) = 1445 AB^2+AC^2=2((\frac{51}{2})^2+(\frac{17}{2})^2)=\boxed{1445}

Well i did it by co ordinate geometry.. was tedious! Good work bro.. this is the best answer :)

Vipul Sharma - 6 years, 6 months ago
Kenny Lau
Jul 4, 2014

NB: Please draw a diagram when viewing this solution, so as to increase the comprehensibility of this solution.

Let D, E, F be the mid-points of the edges opposite to A, B, C respectively.

It is obvious that AGD, BGE, CGF are straight lines.

Since G is the centroid, FG=0.5CG=7.5 and EG=0.5BG=4.

Since the circle passes through G, arcBGC is a semicircle. Therefore, angleBGC is a right angle.

Therefore, angleEGC and angleFGB are also right angles.

Therefore, triangleEGC and triangleFGB are right triangles. (yds)

Therefore, EC = sqrt(241) by the Pythagoras' Theorem.

Similarly, FB = sqrt(120.25)

Since E and F are the mid-points of the sides they are lying on respectively, the length of AB and of AC are trivial.

Therefore the answer is trivial.

Therefore everything is trivial.

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