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## anonymous one year ago Can someone help me with some math questions? I'll fan and medal. :)

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1. anonymous

2. hartnn

do you know how to get projection of one vector on other? tried to solve this?

3. hartnn

Projection of vector a on vector b is $$\LARGE \dfrac{a.b}{|b|}$$

4. anonymous

So it would start off like this?.... (1.6i + 3.3j) (-2.1i -0.5j) = -3.36 -6.93 -1.65 -0.8

5. hartnn

note that $$i . j$$ and $$j.i$$ both = 0 as they are perpendicular vectors!

6. hartnn

so you'll only have 1.6*(-2.1) + 3.3*(-0.5) = ...

7. anonymous

So just to be clear - Whenever you're multiplying the different variables in a vector, the become zero?

8. hartnn

correction* whenever you're taking the dot product of perpendicular vectors, the product = 0 i , j and k are perpendicular vecotrs

9. anonymous

Got it. So... 1.6*(-2.1) + 3.3*(-0.5) = -3.36i -1.65j then you divide that answer by (-2.1i -0.5j) (-3.36i -1.65j) / (-2.1i -0.5j) =...

10. anonymous

Give me just a second on the division. I think I did something wrong.

11. hartnn

so your u.v will be = 1.6*(-2.1) + 3.3*(-0.5) = ... its a numeric value, because its a dot product it won't have i's and j's in it!

12. hartnn

and for the denominator its magnitude of v = $$\sqrt {2.1^2+0.5^2} = ..$$ there is no need of doing complex divisions :)

13. hartnn

1.6*(-2.1) + 3.3*(-0.5) = -5.01

14. anonymous

So the numerator is -5.01 and the denominator is about 2.16 and you would divide that?

15. hartnn

thats right that will give you the magnitude of the projection vector :)

16. anonymous

About -2.32

17. hartnn

thats the magnitude, now you only need the direction of projection vector and since projection is along the vector 'v' its direction will be same as that of vector v !

18. hartnn

for that, you'll need to find the unit vector in the direction of v and then mutiply the above answer to it.

19. anonymous

Wouldn't "b" be the direction? (-2.1i -0.5j)

20. hartnn

you need projection of vector u along vector 'v' so its direction will be along vector 'v' whats b and where it came form ?

21. hartnn

unit vector along v = v/ |v| and we already know |v| = 2.16

22. hartnn

so unit vector along v = (-2.1i -0.5j)/ 2.16 and your projection will be -2.32 times the above answer

23. anonymous

(-2.1i -0.5j)/ 2.16 Would that be (-0.97i -0.23)?

24. hartnn

what do u get after multiplying that with -2.32?

25. anonymous

-2.32 (-0.97i -0.23) = (-2.25i -0.53j)

26. hartnn

two negatives become positive! and you can approximate 2.25 to 2.3 and 0.53 to 0.5 :)

27. hartnn

-2.32 * -0.97 = +2.3 -2.32 * -0.23 = +0.5

28. anonymous

Oops! I didn't even see the double negatives. Thank you for catching that. So the answer would be B. (2.3i +0.5j)

29. hartnn

yes, correct! good work :)

30. anonymous

Thank you for your help. :)

31. hartnn

most welcome ^_^

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