一、利用判定定理(1)同位角相等,两直线平行;(2)内错角相等,两直线平行;(3)同旁内角互补,两直线平行.例1如图1,⊙O是ABC的外接圆,且AB=AC,PA是⊙O的切线,A为切点,割线PBD过圆心,交⊙O于另一点D,试说明PA∥BC.解∵PA是⊙O的切线,∴∠PAB=∠2.∵AB=AC,∴∠1=∠2,∴∠PAB=∠1.∴P First, the use of the theorem (1) is equal to the azimuth angle, two parallel lines; (2) the inner angle is equal, the two lines are parallel; (3) complementary with the side angle, the two straight lines. Example 1 as shown in Figure 1, ⊙O is ABC The circumcircle, and AB = AC, PA is the tangent of ⊙O, A is the cut point, secant PBD over the center of the circle, the intersection of O and another D, try to explain PA ∥ BC. Solution ∵ PA is the tangent of ⊙ O, ∴∠PAB=∠2.∵AB=AC,∴∠1=∠2,∴∠PAB=∠1.∴P