secant

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April 12th 2010, 03:49 PM
euclid2

secant

Determine an expression, in simplified form, for the slope of the secant PQ

a) $P(1,3), Q(1+h,f(1+h))$ , where $f(x)=3x^2$

Now, i know which formula to use, which is

$Limh->0 \frac{(x+h)-f(x)}{h}$

i have the solution to this problem, but don't understand where the numbers are coming from so it's useless to me. i don't understand how to plug the numbers into this formula

for the first step it is:

$Limh->0 \frac{3(1+h)^2-3}{h}<br />$

then simply simplifying to get

3h+6

can someone please explain this to me?
April 12th 2010, 03:56 PM
Deadstar

Quote:

Originally Posted by euclid2

Determine an expression, in simplified form, for the slope of the secant PQ

a) $P(1,3), Q(1+h,f(1+h))$ , where $f(x)=3x^2$

Now, i know which formula to use, which is

$Limh->0 \frac{(x+h)-f(x)}{h}$

i have the solution to this problem, but don't understand where the numbers are coming from so it's useless to me. i don't understand how to plug the numbers into this formula

for the first step it is:

$Limh->0 \frac{3(1+h)^2-3}{h}<br />$

then simply simplifying to get

3h+6

can someone please explain this to me?

Formula should be $\lim_{h \to 0} \frac{f(x+h) - f(x)}{h}$

You're taking $x=1$ and hence $1+h$ .

So $f(1) = 1$ and $f(1+h) = 3(1+h)^2$ .

Hence, $\lim_{h \to 0} \frac{f(1+h)-f(1)}{h} = \frac{3(1+h)^2 - 3}{h}= \frac{3h^2 + 6h}{h} = 3h + 6 = 6$ as $h \to 0$
April 12th 2010, 04:15 PM
euclid2

Quote:

Originally Posted by Deadstar

Formula should be $\lim_{h \to 0} \frac{f(x+h) - f(x)}{h}$

You're taking $x=1$ and hence $1+h$ .

So $f(1) = 1$ and $f(1+h) = 3(1+h)^2$ .

Hence, $\lim_{h \to 0} \frac{f(1+h)-f(1)}{h} = \frac{3(1+h)^2 - 3}{h}= \frac{3h^2 + 6h}{h} = 3h + 6 = 6$ as $h \to 0$

so what you're doing is plugging (1+h) into 3x^2 to get 3(1+h)^2 ??

and thanks for the help!!
April 12th 2010, 04:22 PM
Deadstar

Quote:

Originally Posted by euclid2

so what you're doing is plugging (1+h) into 3x^2 to get 3(1+h)^2 ??

and thanks for the help!!

Yeah, see what I think is going on in the question is you are being asked to find the slope of the curve at the point (1,3).

Now you can do this simply by differentiating f(x) (= 6x) and plugging in x=1.

But it seems they want you to use the formula...

$\lim_{h \to 0} \frac{f(x+h) - f(x)}{h}$

Read this to learn more.
Derivative - Wikipedia, the free encyclopedia

Since we are using x=1 (as in P(1,3)).

Plug x into the above formula and just expand it.