- #1
QuietMind
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Homework Statement
Use algebra to show that U(x) = −√x − 1 and L(x) = −√x satisfy the ’funnel condition’ U(x) − L(x) → 0 as x → ∞
Homework Equations
Funnel condition: The two fences come together asymptotically, i.e. U(x) − L(x) is small for large x.
The Attempt at a Solution
I think that the binomial expansion is appropriate here? But I am unclear if it can be used for fractions.
##U(x)-L(x) = -\sqrt{x-1} +\sqrt{x} ##
the first term can be expressed as: (hopefully did the binomial right)
## x^\frac{1}{2} - \frac{1}{2}x^\frac{-1}{2}- \frac{1}{8}x^\frac{-3}{2}...##
so then
##-\sqrt{x-1} +\sqrt{x} = x^\frac{1}{2} -x^\frac{1}{2} + \frac{1}{2}x^\frac{-1}{2} + \frac{1}{8}x^\frac{-3}{2}... ##
the first 2 terms cancel, then the rest go to 0 as x goes to infinity. Does that look right?