Angle Outlier

Geometry Level 4

Circle O O is the circumcircle of A B C \triangle ABC , and O P B C OP\parallel BC . If A = 8 0 \angle A=80^\circ , find C P O \angle CPO , in degrees.

20 10 5 40

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4 solutions

Lemuel Liverosk
May 9, 2016

m B O C = 2 m B A C = 160 ° m\angle BOC=2\cdot m\angle BAC=160° , thus m B C O = 1 2 ( 180 ° m B O C ) = 10 ° m\angle BCO=\frac{1}{2}(180°-m\angle BOC)=10° . m P C O = m C P O m\angle PCO=m\angle CPO and due to parallelism, m C P O = m B C P m\angle CPO=m\angle BCP , then m P C O = m B C P = 1 2 m B C O = 5 ° m\angle PCO=m\angle BCP=\frac{1}{2}m\angle BCO=5° , leading to the answer that m C P O = 5 ° m\angle CPO=5° .

<(boc)=160 and <obc=10 then< pob=10 thats is arc (pB) so <pcb =5 and< p =5

Soumava Pal
Jul 4, 2020

Joining P O PO to meet the circle at X X , and joining B X , P B , P A BX, PB, PA , we get P A B = P X B = P C B = X P C = θ \angle PAB = \angle PXB = \angle PCB = \angle XPC = \theta , where the first two equalities follow from them being angles in the same segment and the last one follows from parallelity of O P OP and B C BC .

Also P X PX being the diameter, we get P B X = 9 0 = P X B + B P X = θ + θ + 8 0 θ = 5 \angle PBX = 90^{\circ} = \angle PXB + \angle BPX = \theta + \theta + 80^{\circ} \implies \theta = 5^{\circ}

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