Isobaric process

Constant $P$. Because the pressure does not change, it comes outside the volume change and the sheet's work expression is exact rather than an average:

Here $W$ is the work done ON the gas, per essential knowledge 9.4.B.1.iii. Expand at constant pressure and $\Delta V > 0$, so $W < 0$ and the gas gives energy up. Compress it and $W > 0$.

On a pressure-volume diagram the path is a horizontal line, so the area beneath it is a rectangle of height $P$ and width equal to the size of $\Delta V$. That area is the magnitude of the work. The sign comes from the direction of travel, which is why 9.4.B.2.ii is careful to say the absolute value of the work equals the area under the curve.

The other two terms follow. For an ideal monatomic gas $U = \frac{3}{2}PV$, so at constant pressure $\Delta U = \frac{3}{2}P\Delta V$. Substituting into $\Delta U = Q + W$ gives $Q = \frac{5}{2}P\Delta V$, so an isobaric expansion always requires heating, with three fifths of the energy supplied going into internal energy and two fifths leaving as work on the surroundings.

Constant pressure is easier to arrange than it sounds. A gas under a freely moving piston carrying a fixed weight sits at whatever pressure balances that weight, whatever the volume does.

The CED lists isobaric among the special cases at 9.4.B.3. Combining these paths into cycles belongs to the PV diagrams guide.