# angle depression

• Mar 19th 2008, 02:04 PM
victorfk06
angle depression
A light house keeper 120 feet above the water sees a boat sailing in a straight line directly toward her. As she watches, the angle of depression to the boat changes from 28 degrees to 43 degrees. How far has the boat traveled during this time?
• Mar 19th 2008, 02:54 PM
Soroban
Hello, victorfk06!

Quote:

A lighthouse keeper 120 feet above the water sees a boat
sailing in a straight line directly toward her.
As she watches, the angle of depression to the boat changes from 28° to 43°.
How far has the boat traveled during this time?

Code:

    A * - - - - - - - - - - -       |  *  28°       | 62°  *       |        *   120 |            *       |              *       |                  *       |                    *       |                        *       * - - - -*- - - - - - - - - *       B  y    C        x        D
In right triangle $ABD\!:\;\;\tan62^o \:=\:\frac{x+y}{120}\quad\Rightarrow\quad x \;=\;120\tan62^o - y$ .[1]

Code:

    A * - - - - - -       |* 43°       | *       |  *   120 |47°*       |    *       |    *       |      *       |      *       * - - - -*       B  y    C
In right triangle $ABC\!:\;\;\tan47^o \:=\:\frac{y}{120} \quad\Rightarrow\quad y \:=\:120\tan47^o$ .[2]

Substitute [2] into [1]: . $x \;=\;120\tan62^o - 120\tan47^o \;=\;97.0029306$

. . Therefore, the boat traveled about 97 feet.

• Mar 19th 2008, 02:58 PM
galactus
There are various approaches, but the law of tangents works OK.

$120(tan(62)-tan(47))$