Question 1012446
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An 18-foot ladder leans against the wall of a building. The base of the ladder is 9 feet from the building on level ground. 
How many feet up the wall, is the top of the ladder? Leave your answer in simplest radical form.
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From the right-angled triangle you have


h = {{{sqrt(18^2-9^2)}}} = {{{9*sqrt(3)}}} feet


By the way, this right-angled triangle is 30-60-90 degrees.