急! 數學解圖

圖片參考：http://h.imagehost.org/0002/20100612132.jpg


cosㄥACB = (BC^2 + AC^2 - AB^2) / (2 BC AC)

cosㄥACB = (7^2 + 6^2 - 8^2) / (2*7*6) = 1/4

;

cosㄥACD = cos(180° - ㄥACB) = - cosㄥACB = - 1/4

(AC^2 + DC^2 - AD^2) / (2 AC DC) = -1/4

(6^2 + 5^2 - x^2) / (2*6*5) = -1/4

(61 - x^2) / 60 = -1/4

61 - x^2 = - 15

x^2 = 76

x = 2√19