I had to show that the angle between the planes is approx 43 degrees. The two vectors is <1,3,1> and <2,-4,-3> using the cos theta = (a dot b) / (|a||b|) theta was approx 136. I subtracted 180 and got -43. Does that show that the angle is 43 degrees even though it's negative? and why does subtracting 180 work?
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You should be subtracting FROM 180, i.e. 180 - 136 = 44.