this question is from chapter 16 integrals and vector fields, thomas calculus. please explain all the concepts and theory behind the terms .

20. Let S be an oriented surface parametrizationefine the notation do = r, du X r, du so that do is a vector normal to the surface. Also, the magnitude do = do is the element of sur- face area (by Equation 5 in Section 16.5). Derive the identity do = (EG - F2) 1/2 du du where E = r, F= rry, and G = ry|2