Let T be the surface tension of a soap solution.
The initial surface area of soap bubble = 0
The final surface area of soap bubble = 2 × 4πr2
∴ The increase in surface area = 2 × 4πr2
The work done in blowing the soap bubble is
W = surface tension x increase in surface area
=T × 2 × 4πr2 = 8πr2T