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Geometry Level 2

In triangle A B C ABC , A E , F C , D B AE,FC,DB are angle bisectors and quadrilateral F O E B FOEB is a cylic quadrilateral. Find the measure of angle O F G OFG .


The answer is 30.

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2 solutions

Mehul Chaturvedi
Oct 12, 2014

L e t C A B = 2 a A C B = 2 c A B C = 2 b W e K n o w a + 2 c = A E B ( E x t e r i o r a n g l e o f t r i a n g l e ) S i m i l a r i l y c + 2 a = O F B B u t a s q u a d r i l a t e r a l F O E B i s c y c l i c T h e t e f o r e O F B + O E B = 180 ( P r o p e r t y o f c y c l i c q u a d . ) T h e r e f o r e a + 2 c + 2 a + c = 180 3 ( a + c ) = 180 a + c = 60 N o w i n T r i a n g l e A B C A + B + C = 180 s o , B = 180 2 ( 60 ) B = 60 s o , b = 30 B u t c = O F E ( f r o m s a m e c h o r d ) T h e r e f o r e O F E = 30 Let\angle CAB=2a\\ \angle ACB=2c\\ \angle ABC=2b\\ We\quad Know\quad \angle a+2\angle c=\angle AEB(Exterior\quad angle\quad of\quad triangle)\\ Similarily\quad \angle c+2\angle a=\angle OFB\\ But\quad as\quad quadrilateral\quad FOEB\quad is\quad cyclic\quad Thetefore\angle OFB+\angle OEB=180(Property\quad of\quad cyclic\quad quad.)\\ Therefore\\ \angle a+2\angle c+2\angle a+\angle c=180\\ 3(\angle a+\angle c)=180\\ \angle a+\angle c=60\\ Now\quad in\quad Triangle\quad ABC\quad \angle A+\angle B+\angle C=180\\ so,\angle B=180-2(60)\\ \angle B=60\quad so,\quad \angle b=30\\ But\quad \angle c=\angle OFE(from\quad same\quad chord)\\ Therefore\quad \angle OFE=30

Dhruv G
Feb 28, 2015

The problem can be solved intuitively by considering ABC as an equilateral triangle(a general rule must apply to a special case). Thus, FBE=60 deg FOE=120 deg (FOEB is cyclic in given as well as in case of equilateral triangle) OFE=OEF since OF=OE= median/3 so, OFG=30 deg=(180-120)/2 deg

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