Typo in solution manual in Activity 10.7.6

There's a sign error when you find the derivative of f(t, 4-t); the result should be -4t+14 rather than 8t+14. Therefore there is a critical point along the hypotenuse at (7/2, 1/2). (This accords with the figure, which shows a peak / valley along each of the boundary edges.)

f(7/2, 1/2) = 1/2, so this doesn't change the conclusion about where's the absolute max and absolute min on the region.