From the picture, we see that this boils down to finding the length of a side of a right triangle. The angle that we know about is 45^ and we know that the length of the side adjacent to the angle is 100 feet. We want to know about the length of the side that is opposite to the angle.
The tangent of an angle alpha is the ratio of the length of the opposite side to the length of the adjacent side
tan45^=x/100
1=x/100
x=100
So, the river is 100 feet across.
Now answer the following question(s).
1. Name the trigonometric ratios that was/were used to evaluate the width of the river?
2. List the most common units angles can be measured in.