Question

A surveyor measures the distance across a straight river by the following method. Starting directly across from a tree on the opposite bank, she walks 90 m along the riverbank to establish a baseline. Then she sights across to the tree. The angle from her baseline to the tree is 50.0°. How wide is the river?

Answer 1

Draw a right-triangle with an angle of 50° to the base which measures b=90 m.
The height of the triangle is the width of the river, w.
so w=b*tan(50°)

Answer 2

45