三角形ABC中,a=4、b=6、c=120度、求sinA
问题描述：
三角形ABC中,a=4、b=6、c=120度、求sinA
答
cosC = (a² + b² - c²)/2ab
-1/2 = (4² + 6² - c²)/(2 × 4 × 6)
c = 2√19
a/sinA = c/sinC
4/sinA = 2√19/(√3/2)
sinA = √57/19