The distance between the observer and building is 24 feet.

Let the angle of elevation is .

Height of the building is .

The elevator moves up from the ground at rate 4 ft/sec.

The person moves towards building at rate 0.75 ft/sec.

Find the angle of elevation increases or decreases after 10 sec.

Now draw a triangle with the given specifications.

From the above triangle .

Apply derivative on each side with respect to each side.

The distance between man and building after 10 sec.

The distance traveled by person after 10 sec is 7.5.

The remaining distance between the person and building is .

The position of the elevator after 10 sec.

Substitute and

0.1212 is positive and is also positive for all values of .

So is positive.

Therefore the angle of elevation increases.