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Heres the text :Find the angle of vectors a=3p+2q and b=p+5q and if p and q are orthogonal unit ? ... ?

ok the formula is pretty simple cos=a*b/!a! * !b! =

But once i progress and I get 3p square + 15pq + 2qp +10q square ,now I see here that 15pq and 2qp are eliminated is just BEEEYOOOND MEEE I MEAN hows that possible ,thanks in advance ?