# I need help! Can someone help me with this problem?

1. Apr 19, 2007

### princiebebe57

What is the length of the resultant vector of combining 2A - 3B where vector A= 4i + 3j and vector B= 9i + 8j?

2. Apr 19, 2007

### J77

First thing is to multiply the vectors by the given amounts... post your result.

3. Apr 19, 2007

### princiebebe57

is it -19i + 30j?

4. Apr 19, 2007

### J77

No, what's 2A and 3B?

5. Apr 19, 2007

### princiebebe57

2A = 8i + 6j
3B = 27i + 24j

is that correct???

6. Apr 19, 2007

### J77

Yep -- now do the subtraction.

7. Apr 19, 2007

### princiebebe57

so (8i + 6j)-(27i + 24j)?

8. Apr 19, 2007

### J77

Yeah. What's it equal?

9. Apr 19, 2007

### princiebebe57

is it -19i - 18j?

10. Apr 19, 2007

### J77

Yeah -- and the length (magnitude) is...

11. Apr 19, 2007

### princiebebe57

i'm stuck at that!

12. Apr 19, 2007

### cristo

Staff Emeritus
The magnitude of a vector $x\vec{i}+y\vec{j}=\sqrt{x^2+y^2}$. So, what does give you in your case?

Last edited: Apr 19, 2007
13. Apr 19, 2007

### princiebebe57

so is it 26.172?

14. Apr 19, 2007

### J77

Yep -- to 3 decimal places.

15. Apr 19, 2007

### princiebebe57

Thank you so much!!

16. Apr 19, 2007

### Edgardo

Hello princiebebe57,

you may find this website here interesting. It contains some questions and answers on vectors.
If you want to remember the formula for the length of a vector, it helps thinking of the Pythagorean theorem.