# what does the ar in arcosB stand for?

regarding to the cosine formula solving with all sides and no angles?

arcosB in the formula given where is it derived?

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en.wikipedia.org/wiki/… – Daniel R Dec 5 '13 at 20:47
It's actually an arc- prefix, but arccosine is awkward so we sometimes drop one of the c's. – vadim123 Dec 5 '13 at 20:48
@vadim123 I haven't seen arcos though, it's normally written $\arccos$ afaik. – Daniel R Dec 5 '13 at 20:49
...trying to be diplomatic. – vadim123 Dec 5 '13 at 20:51
$$\;\; : - ) \;$$ – Daniel R Dec 5 '13 at 20:55

You probably started with $b^2=a^2+c^2-2ac\cos B$, rearranged that to $\cos B=\frac {a^2+c^2-b^2}{2ac}$ and then saw $B=\arccos(\frac {a^2+c^2-b^2}{2ac})$ It is the inverse function to $\cos$, sometimes written $\cos^{-1}$. It says $B$ is the angle whose cosine is $\frac {a^2+c^2-b^2}{2ac}$