若sin(a+b)=二分之一,sin(a-b)=三分之一,则tan a/tan b为?A.5 B.-1 C.6 D1/6

问题描述:

若sin(a+b)=二分之一,sin(a-b)=三分之一,则tan a/tan b为?A.5 B.-1 C.6 D1/6

sin(a+b)=1/2 展开 为 sina cosb+sinb cosa =1/2 ------------------ (1)sin(a-b)=1/2展开 为 sina cosb-sinb cosa =1/3 -------------------(2)(1)+(2) :2sinacosb= 5/6 -----------------------------------(3)(1)...