题目

如果1×2×3+2×3×4+3×4×5+…+n(n+1)(n+2)=n(n+1)(n+a)(n+b)对一切正整数n都成立， a，b的值应该等于（    ）A.a=1,b=3                     B.a=-1,b=1C.a=1,b=2                     D.a=2,b=3 答案：思路解析：令n=1,2，得到关于a、b的方程组，解得即可.答案：D— How many rectangles are there? — A.Yes, there is.B.No, there aren't.C.There are three rectangles.