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.