Answer: Choice A) [tex]\cos^{-1} \frac{5}{26}[/tex]
You could also write [tex]\cos^{-1} \left(\frac{5}{26}\right)[/tex] which is the same as writing [tex]\arccos\left(\frac{5}{26}\right)[/tex]
Arccos stands for "arccosine", which is effectively the same as inverse sine. The notation [tex]\cos^{-1}[/tex] means inverse cosine.
We use the cosine rule because
cos(angle) = adjacent/hypotenuse.
isolating the angle leads to
angle = arccos(adjacent/hypotenuse)
We can think of arccosine as an operation to undo the cosine function. It's similar to how division is the opposite of multiplication.
