That stands for the surface area element on this surface.

它表示曲面上的面积元素。

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For the surface, pick the Pen Tool and draw a path around the side panel as shown in the image below.

在表面上，使用钢笔工具绘制一个如下图一样包围侧板的路径。

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Employing abrasive machining for the form of the gear and intellective-control carburizing and quenching technology for the surface, the gear is with high intensity, strong tenacity and wearability.

齿轮齿形采用磨齿加工，齿面采用智能控制渗碳淬火技术，齿轮的强度高、韧性和耐磨性好。

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Are they ready with a press release proclaiming a sell-out weekend for the Surface Pro.

FORBES: Microsoft's PR Team Misfires As The Surface Pro Launches

At the equator, it takes about 25 days for the surface of the Sun to rotate on its axis.

BBC: Sun has strange 'spin cycle'

For the Surface Pro, there is also a built-in downside to their display technology because of Windows 8, which I note below.

FORBES: Why You Want The Microsoft Surface Pro (And Why You Don't)