有钝角三角形ABC,钝角为120度,AD平分该钝角,求证1/AD=1/AB+1/AC

过D作DE‖AC交AB于E
则由钝角为120度,AD平分该钝角
可得三角形ADE是等边三角形
∴BE/AE=BD/CD
(AB-AE)/AE=BD/CD
(AB-AD)/AD=BD/CD
AB/AD-1=BD/CD
又BD/CD=AB/AC
∴AB/AD-1=AB/AC
1/AD-1/AB=1/AC
1/AD=1/AB+1/AC