Posted by **Joanie** on Thursday, January 29, 2009 at 9:48pm.

Given vectors u=7i + 3j and v = -4i + 3j, find 5u-4v in terms of unit vectors.

I am just beginning the study of vectors and have this problem. Could someone help? Thanks.

- PreCalculus -
**Reiny**, Thursday, January 29, 2009 at 9:56pm
handle them just like algebraic expressions

u=7i + 3j and v = -4i + 3j

then 5u - 4v

= 5(7i+3j) - 4(-4i+3j)

= 35i + 15j + 16i - 12j

= 51i + 3j

- PreCalculus -
**Joanie**, Friday, January 30, 2009 at 1:54pm
Dear Reiny,

Thank you for helping me. Could you check an answer for me?

Use a metric ruler and a protractor to find 2 a (vector) - 2 b (vector). Then find the magnitude and amplitude of the resultant.

Vector a is 4 cm and the angle is 60 degrees. Vector b is 1 centimeter long and the angle is 135 degrees.

My answer for magnitude was 4 cm with the angle being 31 degrees. Is this correct?

- PreCalculus -
**drwls**, Friday, January 30, 2009 at 2:56pm
vector a = 4 cos 60 i + 4 sin 60 j

= 2 i + 0.866 j

vector b = -2.828 i + 2.828 j

"i" and "j" indicate the components along the x and y axes. It is a commonly used vector notation

a - b = -0.828 i + 3.694j

magnitude of a - b = 3.785

magnitude of 2 (a-b) = 7.571

Your angle is also wrong

- PreCalculus -
**Joanie**, Friday, January 30, 2009 at 5:22pm
Hi drwls--

I have just begun the study of vectors. I am given the answer in multiple choice--The answer you gave me is 7.571. The choices I have are 8cm;41 degrees; 9cm;38degrees;6cm,36degrees, and 4cm,31 degrees.

Your answer is closest to 8 cm. Is that the one I should select? Thanks.

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