I met a proof problem that is as follows.(adsbygoogle = window.adsbygoogle || []).push({});

##\bf a = ∫_S d \bf a##, where S is the surface and ##\bf a ##is the vector area of it.

Please proof that ##\bf a = \frac{1}2\oint \! \bf r \times d\bf l##, where integration is around the boundary line.

Any help would be very appreciated!

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# Surface area and line integral

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