Answer
Over time, we gain more knowledge at a slower rate as we approach the maximum level.
Since the slope of the graph is decreasing, then $K''(t) \lt 0$. Therefore, the graph is concave downward.
In the graph of the learning curve, we can see that the graph is concave downward.
Thus, $K(3) - K(2)$ would be larger than $K(8) - K(7)$.
Work Step by Step
Initially, when we first start studying, we can gain new knowledge quite quickly.
Over time, we gain more knowledge at a slower rate as we approach the maximum level.
Therefore, the slope of the $K(t)$ graph has a steep positive slope initially and then gradually becomes less positive. Since the slope of the graph is decreasing, that is $K'(t)$ is decreasing, then $K''(t) \lt 0$. Therefore, the graph is concave downward.
In the graph of the learning curve, we can see that the graph is concave downward.
Thus, $K(3) - K(2)$ would be larger than $K(8) - K(7)$.