Question

A surveyor sights the far bank of a river at an angle of 110° to the near bank. She then moves 75 feet upriver and sights the same point on the far bank of the river at an angle of 150°. What is the shortest distance across the river? Please help. Will give brainliest.

Answer 1

Answer:

Correct answer is 54.82 ft.

Step-by-step explanation:

First of all, let us label the diagram and do the construction as per the attached answer image.

Let us consider $\triangle ABC$:

$\angle B = 90^\circ\\\angle ACB = 180-110 = 70^\circ$

Let side AB = d ft and let side BC = x ft

We need to find AB to find the shortest distance across the river.

Using trigonometric identity of tangent:

$tan\theta = \dfrac{Perpendicular}{Base}$

$tan 70 = \dfrac{d}{x}\\\Rightarrow x = \dfrac{d}{tan70} = 0.36d ..... (1)$

Now, let us have a look at another right angled triangle ABD:

Let us consider $\triangle ABC$:

$\angle B = 90^\circ\\\angle ADB = 180-150 = 30^\circ$

side AB = d ft and side BD = x+75 ft

Using trigonometric identity of tangent:

$tan\theta = \dfrac{Perpendicular}{Base}$

$tan 30 = \dfrac{d}{x+75}\\\text{Using equation (1)}:\\0.577 = \dfrac{d}{0.36d+75}\\\Rightarrow 0.207d + 43.27 = d\\\Rightarrow 0.793d = 43.27\\\Rightarrow d \approx 54.82\ ft$

Correct answer is 54.82 ft.