Find the trigonometric answer

Cos 108° ≈

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?

Which will form a composite function?

How many liters of peroxide should be added to 12 liters of an 8% peroxide solution so that the resulting solution contains 16% peroxide?   original added new amount of peroxide 0.96 liters x 0.96 + x amount of solution 12 liters x 12 + x 0.18 liter 1.14 liters 1.92 liters 2.10 liters