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**skeske1234** The vectors a=3i-4j-k and b=2i+3j-6k are the diagonals of a parallelogram. Show that this parallelogram is a rhombus, and determine the lengths of the sides and the angles between the sides.

So I found part A, the lengths of the sides, but I am having trouble finding part B, the angles between the sides.

For part A, the answer I have is [5(3)^0.5]/2, which is correct.

For part B, I think i have to use the formula

cos(theta)=(a.b)/(|a||b|), but when I plug in the given to the formula, I do not get the answer (same as back).

cos(theta)=(6-12+6)/(5(3)^0.5)

theta=90degrees

what did i do wrong?