Obtuse/Acute/Right angles for dot product (1 Viewer)

[SadCeliac]

We know that a.b = 0 is 90degrees, and a.b<0 is obtuse and a.b>0 is acute, but how do we prove this?? I can show it by plugging in values, but is there a formal way of showing this?

e.g. for this question (which isn't hard it's just how do we prove that a.b>0 is obtuse if we are asked to?)


@carrotsss I feel like you would know lol

 

[synthesisFR]

Is it not just angle between two vectors formula but if it’s obtuse u just state that cos theta is less than zero and the magnitude of the vectors r positive so the dot product is zero