Off Center Angle

Geometry Level 2

The points A , B , C A, B, C are on a circumference, where O O is the center, and P P is the intersection point of A C AC and B O BO , as shown above.

If A B P = 4 9 \angle ABP = 49^{\circ} and P C O = 1 7 \angle PCO = 17^{\circ} , what is the value of B P C \angle BPC in degrees?


The answer is 81.

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4 solutions

Since A P B + B P C = 18 0 \angle APB + \angle BPC = 180^{\circ} , then B P C = B A P + A B P = B A P + 4 9 \angle BPC = \angle BAP + \angle ABP = \angle BAP + 49^{\circ} .

Thus, we are attempting to find B A P \angle BAP for a solution, and we can do so by extending the O C OC line to meet point D D on the circumference before joining it with point B B , as shown below:

According to Inscribed Angles Theorem , B A P = B D O \angle BAP = \angle BDO , and because B D O \triangle BDO is an isosceles triangle, D B O = B D O \angle DBO = \angle BDO .

Furthermore, from Inscribed Angles Theorem , A B D = A C D = 1 7 \angle ABD = \angle ACD = 17^{\circ} .

Thus, A B P = 4 9 = A B D + D B O = 1 7 + B A P \angle ABP = 49^{\circ} = \angle ABD + \angle DBO = 17^{\circ} + \angle BAP .

Hence, B A P = 4 9 1 7 = 3 2 \angle BAP = 49^{\circ} -17^{\circ} = 32^{\circ} .

Finally, B P C = 4 9 + 3 2 = 8 1 \angle BPC = 49^{\circ} + 32^{\circ} = 81^{\circ} .

@Worranat Pakornrat Hats off , sir !👌🙇

Toshit Jain - 4 years, 3 months ago

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Thanks! :)

Worranat Pakornrat - 4 years, 3 months ago

Sorry, I really find this confusing... only if someone had a video in this

Evan The Carrot - 1 year ago

Joining OA, We see that both the blue angles are equal and so are the 2 yellow angles. (By isoceles triangle theorem since radii are equal)

So , Using exterior angle theorem in triangle P A B PAB , We get R e d = 2 Y e l l o w B l u e Red=2Yellow-Blue

Thus R e d = 2 49 17 = 98 17 = 81 Red = 2*49-17=98-17=81

Sudhamsh Suraj
Mar 8, 2017

Upper image for diagram

Lower image for solution.. easy to understand

1) OBC Is an isosceles triangle.

2) 2(Angle BAC) = Angle BOC.

Let us take Angle BAC=θ.

So angle BOC =2θ.

Took me a few mins how you got 90 - theta and 72 - theta, but that's from your last remark :) nice solution.

Peter van der Linden - 4 years, 3 months ago

Actually it is 73-theta , not 72 - theta @Peter van der Linden

Sudhamsh Suraj - 4 years, 3 months ago
Roger Erisman
Mar 8, 2017

Since BAP and BOC cut the same arc, BAP = 1/2 * BOC

BPC = BAP + 49 = BOC + 17 because an exterior angle = sum of the two non adjacent angles

Substituting 1/2*BOC + 49 = BOC + 17

Solving BOC = 64

64 + 17 = 81 degrees

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