题目

D.选修4 – 5 不等式证明选讲 设a,b,c为正实数,求证:a3 + b3 + c3 + ≥2. 答案：D.选修4 – 5 不等式证明选讲 设a,b,c为正实数,求证:a3 + b3 + c3 + ≥2. 证明 因为a,b,c为正实数,所以a3 + b3 + c3≥3 = 3abc>0…………………………5分 又3abc + ≥2 = 2. 所以a3 + b3 + c3 + ≥2.…………………………………………………………………10分Earth is believed ______ by an object the size of Mars at some point in the distant past before itssurface cooled.A．to be hit B．to have hit C．to have been hit D．to hit