# Questions on unit vectors

I)Find the angle between (I+j )and (j+k)?
II)Find the angle between (I+j) and (j-k)?

 Unknown008 Posts: 8,147, Reputation: 3745 Uber Member #2 Aug 25, 2009, 11:38 AM

I did that in maths, using the dot product formula;

ab.cb = |ab||cb|cos A

Let ab be your vector i+j and cb your vector j+k. Their vectors are:

$$$1 \\ 1 \\ 0$$$ and $$$0 \\ 1\\ 1$$$

Now, the dot product, multiply the similar vectors (i direction with i direction, j with j, and k with k)and add them;

$(1\times 0) + (1 \times 1) + (0 \times 1) = |\sqrt{1^2 + 1^2 + 0^2}||\sqrt{0^2 + 1^2 + 1^2}| cos A$

Solve for A, which is your angle. (you should have 30 degrees)

Do the same for the other one.

Hope it helped!

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