# Some maths problem

1. Sep 6, 2005

### IB

Suppose I have sqrt (a^2 - a [delta] d). What do I do? Do I do like this: sqrt (a^2 - a [delta] d) = a - sqrt(a [delta] d)? Thanks.

PS: One more thing. How to write mathematics with latex?

2. Sep 6, 2005

### Learning Curve

What is sqrt (a^2 - a [delta] d) equal to? or are you asking to simplify it?

3. Sep 6, 2005

### IB

Yeah. I was just wondering whether it can still be simplified.

4. Sep 6, 2005

### Hurkyl

Staff Emeritus
Depending on what you want to do, a differential approximation, or maybe a Taylor sum, might be useful.

But as for algebraic manipulation, what you did is wrong. roots and exponents (usually) distribute over multiplication (and division), not addition (and subtraction).

5. Sep 6, 2005

### IB

Could you give an example to demonstrate that? And how can I correct my wrong algebraic manipulation? Thanks.

6. Sep 6, 2005

7. Sep 7, 2005

### VietDao29

Say a = 5, and delta d = 9 / 5.
So:
$$\sqrt{a ^ 2 - a \Delta d} = \sqrt{5 ^ 2 - 5 \times \frac{9}{5}} = \sqrt{25 - 9} = \sqrt{16} = 4$$
And:
$$a - \sqrt{a \Delta d} = 5 - \sqrt{5 \times \frac{9}{5}} = 5 - \sqrt{9} = 5 - 3 = 2$$
And 4 is not 2.
Viet Dao,

8. Sep 7, 2005

### IB

Thanks, VietDao29.