Numerical method c3Watch
Why we have to use radian in f and h
We use radians because that's the natural measurement of an angle. Furthermore, calculus only works in radians and some numerical approximations make use of calculus therefore there needs to be consistency here.
Word of advice: get used to radians at this level, they are really not that hard to understand and are explained just fine at C2.
When differentiating or integrating trigonometric functions, you must use radians. The derivative of sin(x) is only cos(x) if you're in radians. If you try it in degrees, you'll be out by a factor of 180/pi.