圆锥的轴截面是顶角为120°的三角形，如果它的高是3，则此圆锥的侧面积为 ▲
18
分析：利用圆锥的底面周长等于圆锥的侧面展开图的弧长，求出圆锥母线以及底面圆的周长，利用扇形面积公式求解即可．
解答：
解：过点A作AD⊥BC，
∵圆锥的轴截面是顶角为120°的三角形，它的高是3，
∴AD=3，∠DAC=∠BAD=60°，∴∠C=30°，∴AC=6，CD=3
，
∴圆锥的母线为：6，底面半径为：3，圆锥的底面周长为2π×3=6π，
它的侧面展开图的弧长为：6π，
所以它的侧面展开图的面积为：
×6×6=18π，
故答案为：18π．
点评：此题考查了圆锥的侧面展开图的有关计算问题，根据已知得出圆锥的底面周长以及圆锥的母线长是解题关键．
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