Question

What is the value of d/dx(1/x) at x=6 ?​

Answer 1

If you know your derivative rules, then

d/dx [1/x] = -1/x ²

so that when x = 6, the derivative has a value of -1/36.

If you have to use the definition of the derivative, then

d/dx [1/x] = lim {h → 0} (1/(x + h) - 1/x) / h

… = lim {h → 0} (x - (x + h)) / (hx (x + h))

… = lim {h → 0} (-h) / (hx (x + h))

… = lim {h → 0} (-1) / (x (x + h))

… = -1/x ²

and at x = 6, you again get -1/36.

Alternatively, use the definition of the derivative at a point:

d/dx [1/x] (6) = lim {x → 6} (1/x - 1/6) / (x - 6)

… = lim {x → 6} ((6 - x) / (6x)) / (x - 6)

… = lim {x → 6} -(x - 6) / (6x (x - 6))

… = lim {x → 6} (-1) / (6x)

… = -1/36