My teacher had solved an example question related to stress and strain. The question is as follows:
If a hollow glass sphere of inner radius $r$ and thickness $\Delta r ~ (\Delta r \ll r)$ is filled with some gas, then find the maximum pressure the gas can exert before the sphere breaks. Given: the breaking stress for the glass sphere is $S_b$
Below I have attached a picture of the solution that the teacher had provided to us.
Now, I get that the sphere would break when the stress being applied by the gas on the sphere would be equal to the breaking stress, but then why has the teacher taken two different areas? for one side, its $\pi r^2$ and for the other side its $2\pi r \Delta r$. From what I understand stress is internal force per unit area being applied by an elastic object to resist deformation, so shouldn't the area be the same?