Properties of circle................

Geometry Level 3

In the given figure above O is the centre of the circle, BC and CD are two equal chords and O B C = 7 0 \angle OBC = 70 ^ \circ . You have to find sum of B A C + B F D + C E D \angle BAC+\angle BFD+\angle CED (in degrees).


The answer is 80.

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1 solution

Aman Bansal
Mar 30, 2014

Join OC. As OB = OC = Radius of circle, so O B C = O C B = 7 0 \angle OBC = \angle OCB = 70 ^ \circ . Applying angle sum property in \triangle BOC, we get B O C = 4 0 \angle BOC = 40 ^ \circ .

Now, B O C = C O D = 4 0 \angle BOC = \angle COD = 40 ^ \circ {As angle formed by equal chords on the center are equal}.

B A C = 1 2 B O C = 1 2 4 0 = 2 0 \angle BAC = \frac{1}{2}\angle BOC = \frac{1}{2}40 ^ \circ = 20 ^ \circ {As angle made by a chord on the circumference is half the angle made by it on the center}.

B A C = C E D = 2 0 \angle BAC = \angle CED = 20 ^ \circ {As angle formed by equal chords on the circumference are equal}.

B F D = 1 2 B O D = 1 2 8 0 = 4 0 \angle BFD = \frac{1}{2}\angle BOD = \frac{1}{2}80 ^ \circ = 40 ^ \circ . {As angle made by a chord on the circumference is half the angle made by it on the center}

We have to find B A C + B F D + C E D \angle BAC + \angle BFD + \angle CED which is equal to 2 0 + 4 0 + 2 0 20^ \circ + 40^ \circ + 20^ \circ = 8 0 \boxed{80^\circ} and that's our answer.

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