2/3
证明：扇形的面积=R²π/6
设内切圆半径为 r
60°/2=30°
可作出一三角形：两角为30°,90°,两边为 r, R-r
r/(R-r) = 1/2 --->
r=R/3
内切圆面积 = r²π = R²π/9
(R²π/9) / (R²π/6)= 2/3