# Phys... vectors

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Consider the following three vectors: a = 3i + 4j ; b = pi + 2j ; c = mi + nj , where
m, n and p are constants. If:
 a + b = 9i + 6j
 a.c = 10
 b x c = 2k
Show that p = 6, and determine values for m and n.

i got the p=6

as for the rest...what i did so far

b x c = 2k

so (6i x nj) + 2j x mi

6nk + 2mk = 2k...cancel out k's and simplify by dividing through by 2

so

3n + 1m = 1

___

a.c = 10

so

3i.mi + 4j.nj = 10

3m + 4n = 10

i would say solve simulatneously...but

when i do that, the answer doesn't come out to
m = 2 and n = 1

instead, i get m=3.2 and n=1.4

• Phys... vectors -

On the cross product.
ixj=k but jxi=-k. I think you missed the minus

The equation on the dot is correct.

• Phys... vectors -

wow...lol...look at what one little mistake can do

THANKS!~ i got it now

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