OC写的一个关于三角函数的计算器功能 关于分类以及协议， Programming in OC书上的 chapter11中的第四题-The functions sin(), cos(), and tan() are part of the C Standard Library (as scanf ()

is). These functions are declared in the system header file math.h, which is automatically

imported into your program when you import Foundation.h.

You can use these functions to calculate the sine, cosine, or tangent, respectively, of their

double argument, which is expressed in radians. The result is also returned as a double

precision floating-point value. So, you can use this line to calculate the sine of d, with

the angle d expressed in radians:

result = sin (d)

Add a category called Trig to the Calculator class defined in Chapter 6, “Making

Decisions.” Add methods to this category to calculate the sine, cosine, and tangent based

on these declarations:

-(double) sin

-(double) cos

-(double) tan