Answer: c must be 18
Explanation:
From the triangle inequality we know that the sum of any two sides must be larger than the third side. We us the inequality here:
a = 1, b = 18, c unknown
a + c > b -> 1 + c > 18 -> c > 17
b + c > a -> 18 + c > 1 -> c > 0
a + b > c -> 18 + 1 > c -> c < 19
The third side must be greater than 17 and smaller than 19. So the smallest possible (and the only possible) whole-number length for c is 18
