You have designed a screen saver for your computer that is an expanding circle, constantly changing colours.

The circle starts as a dot in the middle of the screen and expands outward. With a 21-in screen, you have a viewing area with a 15-in radius (measured from the centre diagonally down to a corner). The circle reaches the corners in 5 s.

Express the area of the circle as a function of time t in seconds. (Don't worry about the area cut off by the edges of the viewing area.)