Circle
O
is the
circumcircle
of
△
A
B
C
, and
O
P
∥
B
C
. If
∠
A
=
8
0
∘
, find
∠
C
P
O
, in degrees.
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<(boc)=160 and <obc=10 then< pob=10 thats is arc (pB) so <pcb =5 and< p =5
Joining P O to meet the circle at X , and joining B X , P B , P A , we get ∠ P A B = ∠ P X B = ∠ P C B = ∠ X P C = θ , where the first two equalities follow from them being angles in the same segment and the last one follows from parallelity of O P and B C .
Also P X being the diameter, we get ∠ P B X = 9 0 ∘ = ∠ P X B + ∠ B P X = θ + θ + 8 0 ∘ ⟹ θ = 5 ∘
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m ∠ B O C = 2 ⋅ m ∠ B A C = 1 6 0 ° , thus m ∠ B C O = 2 1 ( 1 8 0 ° − m ∠ B O C ) = 1 0 ° . m ∠ P C O = m ∠ C P O and due to parallelism, m ∠ C P O = m ∠ B C P , then m ∠ P C O = m ∠ B C P = 2 1 m ∠ B C O = 5 ° , leading to the answer that m ∠ C P O = 5 ° .