Maths~

21. < fad= < eab = < bcd ( 圓內接四邊形外角)

so, < adc = 28 + < bcd ; < abc = 50 + < bcd ( 三角形外角)

< adc+ < abc =180 (圓內接四邊形對角)

2< bcd + 78 = 180

< bcd = 51

31. < abt = 180 -5x (直線上的鄰角)

as ta=tb(切線性質),
< bat = 180 -5x( 等腰三角的底角)

x+ 2(180-5x) = 180 (三角形內角和)

x = 20

21. 設o 是圓心.

< pao = <pbo = 90 (切線垂直半徑)

< aob = 360 -2(90) - 58 = 122(多邊形內角和)

< acb = 122/2 = 61 ( 圓心角二倍於圓周角)