# Calculating volume easy question

1. Feb 23, 2012

### Miike012

My work is in the paint doc... my only question is... why is the radi 1-x and not 1+x?

My reason for why I think it is 1 - x is in the paint doc, please let me know why I am wrong.. thank you.

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2. Feb 23, 2012

### Staff: Mentor

3. Feb 23, 2012

### Miike012

Try this image.

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4. Feb 23, 2012

### Staff: Mentor

Much better - thanks!

The horizontal distance from a point (x, y) on your exponential curve to the line x = 1 is 1 - x. For most of the interval [-1, 0], the x values are negative, so this distance will generally be larger than 1, but less than 2.

Also, the point is labelled incorrectly. It should be (x, f(x)), not (-x, f(-x)).

5. Feb 23, 2012

### Miike012

it should be (x,f(x)) even though its in the 2nd quad? is this general for all quad that x and f(x) be positive when labeling an arbitrary coordinate ?

6. Feb 23, 2012

### Staff: Mentor

Yes.
You are assuming that x is always positive and -x is always negative - no, this isn't true. You can't tell the sign of a variable by whether it has a + or - in front of it. For example, -b could be positive or negative, depending on the value of b. Similarly, +c could be positive or negative, depending on the value of c. Note that we don't normally write +c, but I'm just trying to make a point.

Think about the x-axis. If x is a number to the left of zero, it's negative. We DO NOT write this as -x.

7. Feb 23, 2012

### Miike012

Thank you so much that really helps.