M到底面距离小于h/2的概率
=正三棱台(正三棱锥下部高为h/2)的体积/正三棱锥S-ABC的体积
=1-(上部高为h/2的正三棱锥体积/正三棱锥S-ABC的体积)
=1-(1/2)³ (体积比等于相似比的立方)
=7/8