Solving a problem in physics section 72 point 4 electr

Solving a problem in physics section 72 point 4 electr

4. There is a flat air condenser, the area of each plate of which is S. What work needs to be done to slowly increase the distance between the plates from x1 to x2, while maintaining unchanged:
a) capacitor charge equal to q;
b) the voltage across the capacitor is equal to U?

Additional information:

Detailed solution. Decorated in Microsoft Word 2003. (Target decided to use formula editor)