What is the length of the altitude of the equilateral triangle below?

To answer this we apply the Pythagorean Theorem, this states that given a right triangle with a hypotenuse -longest side- () and two sides ( and ), then:



So, for your problem, we can see right triangle with a hypotenuse of 8 (), one side equal to 4 () and other side unknown; this means we have the following:



We can easily solve for  (altitude of the equilateral triangle):

 

Hence, the answer is E.