Is f(x)=(x-3)/sqrt(x+3) increasing or decreasing at x=3 ?

1 Answer
Nov 24, 2016

You have to look at the sign of the derivative of the function for x=3

Explanation:

f(x) = (x-3)/sqrt(x+3) = (x-3)(x+3)^(-1/2)

(df)/(dx) = (x+3)^(-1/2) -1/2 (x-3)(x+3)^(-3/2)

(df)/(dx) |_(x=3) = 1/sqrt(6) > 0

So the function is increasing.

On the other hand it is easy to see that x=3 is a zero of f(x) and as the denominator is always positive and the numerator is negative for x<3 and positive for x>3 the function must be increasing.