Problem:
Real numbers a and b are chosen with 1<a<b such that no triangle with positive area has side lengths 1,a, and b or b1β,a1β, and 1 . What is the smallest possible value of b ?
Answer Choices:
A. 23+3ββ
B. 25β
C. 23+5ββ
D. 23+6ββ
E. 3 Solution:
There is a triangle with side lengths 1,a, and b if and only if a>bβ1. There is a triangle with side lengths b1β,a1β, and 1 if and only if a1β>1βb1β, that is, a<bβ1bβ. Therefore there are no such triangles if and only if bβ1β₯aβ₯bβ1bβ. The smallest possible value of b satisfies bβ1=bβ1bβ, or b2β3b+1=0. The solution with b>1 is 21β(3+5β)β. The corresponding value of a is 21β(1+5β).