I think I have made an error somewhere here as you cannot take the square of a negative in real terms. See below.

U = (-1, -1, -1), V = (3, 1, 0).

U . V = (-1, -1, -1) . (3, 1, 0) = ( (-3) + (-1) + 0 ) = -4.

||U|| SQRT( (-1)^2 + (-1)^2 + (-1)^2 ) =SQRT( (-3) ).

||V|| SQRT( 3^2 + 1^2 + 0^2 ) = SQRT( 10 ).

Cos THETA = U . V / ||U|| ||V|| = -4 /SQRT((-3))* SQRT(10)

THETA Cos^-1 ( -4 / SQRT((-3)) * SQRT(10))

I am thinking this angle is obtuse.

I'm really sorry for the presentation of this but I'm not sure how the tags work. If it's too much of a mess I can try and work out how to use the tags.

Thanks,