函数z=ln(x2+y2)在点M(3,4)处沿z的梯度方向的方向导数为()。
参考答案:
解析:
[分析]:
There are only _______ natural resources as there were thirty years ago. [ ]
A. as half much
B. as half many
C. half as much
D. half as many
在小型建筑工地,常用简易的起重设备竖直吊运建筑材料,其工作原理相当于如图所示的滑轮组。某次将总重G为4000 N的砖块匀速吊运到高为10 m的楼上,用时40 s,卷扬机提供的拉力F为2500 N。求在此过程中:
(1)有用功;
(2)拉力F的功和功率;
(3)滑轮组的机械效率。