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## #1 2017-08-22 07:08:24

Member
From: Planet Mars
Registered: 2016-11-15
Posts: 725

### Why is it called f(x+Δx)?

I was learning differential calculus from mif when I saw this:

Why is the above point called f(x+Δx)?

Practice makes a man perfect.
There is no substitute to hard work
All of us do not have equal talents but everybody has equal oppurtunities to build their talents.-APJ Abdul Kalam

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## #2 2017-08-22 08:50:38

zetafunc
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Registered: 2014-05-21
Posts: 2,109
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### Re: Why is it called f(x+Δx)?

The first point is
. The second point is
.

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## #3 2017-08-22 19:32:16

bob bundy
Registered: 2010-06-20
Posts: 8,139

### Re: Why is it called f(x+Δx)?

In differential calculus, we are trying to develop a rule for how the gradient of a function varies with x.  The usual method for working this out is this:

(1) Pick a point on the curve for the function, say ( x, f(x) )

(2) Move to a point close to ( x, f(x) ).  The way this is written is ( x + Δx,  f(x+Δx) ) meaning ("x plus a little bit more in the x direction", "value of the function at this new value of x")

note: Δx should be regarded as a single variable, not as two variables multiplied together.  Thus Δy/Δx must not be simplified to y/x by cancelling! This should be considered as "a little bit in the y direction"/"a little bit in the x direction"

(3) The gradient of the chord joining these points is then considered, simplifying where possible.

(4) Then Δx is reduced towards zero, to see if the gradient found in step (3) tends towards a recognisable gradient function.

(5) This gradient function is usually written dy/dx.  Again these two, dy and dx, should be treated as two single variables so the 'd's cannot be cancelled.

Some texts avoid this confusion by using x + h and f(x+h) where 'h' is a small amount in the x direction.

Hope that helps,

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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## #4 2017-08-22 22:21:23

Member
From: Planet Mars
Registered: 2016-11-15
Posts: 725

### Re: Why is it called f(x+Δx)?

Thnx bob and zetafunc.

Practice makes a man perfect.
There is no substitute to hard work
All of us do not have equal talents but everybody has equal oppurtunities to build their talents.-APJ Abdul Kalam

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