Question

已知cosa-sina=-二分之一,则sinacosa的值为

Answer 1

由cosa - sina = -1/2 两边平方得：
cos^2 a - 2sinacosa + sin^2 a = 1/4
- 2sinacosa = 1/4 - (cos^2 a + cos^2 a )
= 1/4 - 1
= -3/4
所以 sinacosa = 3/8