Asked by thomas

The number of diagonals, d, in a polygon of n sides is given by the formula d=n^2-3n/2. If a polygon has 90 diagonals, how many sides has it?

a. 8
b. 10
c. 12
d. 15
e. 20

please answer and explain
Answered by Steve
just plug in d=90 and solve

(n^2-3n)/2 - 90 = 0

quadratics are easy, right?
Answered by thomas
I don't think it is easy, can you explain how to do this problem?
Answered by Steve
You have a formula for finding d, given n. You are given d, so you need to find n.
(n^2-3n)/2 = 90
n^2-3n = 180
n^2 - 3n - 180 = 0
(n-15)(n+12) = 0
n = 15 or -12

-12 is not a reasonable answer, so n=15 is the solution.
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