hi how can the following be proved using integral methods:

a) prove surface area of sphere, radius a, is

b) prove area of a disk, radius a, is

c) prove volume of ball, radius a, is

d) prove volume of axisymmetric cone of height h and base with radius a, is

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i think my working of (a) is correct:

working of (a):|S| =

use spherical co-ordinates:

so:

|S| = = = =

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how can i do the rest please. and what integration methods should I be using for each? thnx xxxx