已知tanα=-1/2,求1+sin^2α-3sinαcosα的值

首先知道sin^2α+cos^2α＝1,则 原式=（1+sin^2α-3sinαcosα）/1 =分子分母同除以cos^2α =(1/cos^2α +tan^2α-3tanα)/(1/cos^2α) =/ =(tan^2α+1+tan^2α-3tanα)/(tan^2α+1) =(2tan^2α+1-3tanα)/(tan^2α+...