Answer: 16.8 feet
Note: your teacher may not want you to enter "feet" and instead may just want the number only.
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Work Shown:
tan(angle) = opposite/adjacent
tan(40) = h/20
20*tan(40) = h
h = 20*tan(40)
h = 16.7819926 which is approximate
h = 16.8
The streetlamp is approximately 16.8 ft tall.
When using your calculator, make sure it's in degree mode. One way to check is to compute something like tan(45) and you should get 1 as a result.
Explanation: