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1. Suppose the surface is defined by z = x2 + y2,
and we use x and y as our parameters.
Then w = -2xi -2yj + k;
Its magnitude (used in the area measure), is (4x2+4y2
+1)1/2.
2. Suppose the surface is defined by z = sinx siny and x and y are parameters.
Then w = -cosx sinyi - sinx cosyj + k, and its magnitude
is (cos2x sin2y + sin2xcos2y + 1)1/2.