PC=PD=PE，

Points c, d and e are three * * * circles with p as the center.

From the angle of circle theorem: ∠ DPE = 2 ∠ ECD = 2y.

∠BDP=90，∠BEP=90

∴∠B+∠DPE= 180

∠B= 180 -2y

( 180 ? 2y ) / 360 * 2πR =πR(90？ y )/90