# Use the definition of Derivative to find f`(x)

• Oct 2nd 2009, 01:29 PM
valeriegriff
Use the definition of Derivative to find f`(x)
1. f(x) = 4
(√5 - x)

Note: 5 - x is all squared.

2. f(x) = 2x
x - 2

3. f(x) =
√x + 2
3
• Oct 2nd 2009, 02:08 PM
HallsofIvy
Quote:

Originally Posted by valeriegriff
1. f(x) = 4
(√5 - x)

Note: 5 - x is all squared.

Is it all squared or all in the square root? A clearer way of writing that would be 4/√(5- x). Use parentheses!

That is equal to \$\displaystyle 4(5- x)^{-1/2}\$. Use the "power rule" (derivative of \$\displaystyle x^n\$ is \$\displaystyle n x^{n-1}\$ and the chain rule.

Quote:

[SIZE=5]
2. f(x) = 2x
x - 2

f(x)= 2x/(x- 2). Use the quotient rule!

Quote:

3. f(x) = √x + 2
3

\$\displaystyle (1/3)(x+2)^{1/2}\$

Again, "power rule" and "chain rule".
• Oct 8th 2009, 04:04 PM
valeriegriff
i understand the concept of rules. but the problem i am having with these questions is that i was asked to find the derivative using the definition. i know the definition is
f`(x) = f( x + h) - f(x) Żall over- h

but when i place the equations into the definition i cannot get the same answer as the rules. there is something i am doing wrong and i dont know what..