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Answer: 
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n starts at 1, and n is a positive whole number (1,2,3,...)
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Explanation:
The sequence is arithmetic with first term 40 and common difference 10. Meaning we add 10 to each term to get the next one.
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a1 = 40 = first term
d = 10 = common difference

is the general nth term of this arithmetic sequence
Plug in n = 1 and you should get 
Plug in n = 2 and you should get 
and so on
The height of the triangular base of the pyramid is calculated through the equation,
h = (cos x)(18 in)
where h is height and x is half of the angle of the triangle. Since the triangle is equilateral, the value of x is 30°. Substituting,
h = (cos 30°)(18 in)
h = 15.59 in
Thus, the height of the base is approximately 15.59 inches.
35 is the number you can remove so you can still have the same median