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一些数学计算的公式的具体实现是放在math.h里,具体有：
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double sin (double x); x的正弦值
double cos (double x); x的余弦值
double tan (double x); x的正切值
double asin (double x); 结果介于[-PI/2, PI/2]，x值域为[-1,1]
double acos (double x); 结果介于[0, PI],x值域为[-1,1]
double atan (double x); 反正切(主值), 结果介于[-PI/2, PI/2]
double atan2 (double y, double x); 反正切(整圆值), 结果介于[-PI, PI]
double sinh (double x); x的双曲正弦值
double cosh (double x); x的双曲余弦值
double tanh (double x); x的双曲正切值
double exp (double x); 幂函数e^x
double pow (double x, double y); x^y，如果x=0且y=0,或者x0且y不是整型数，将产生定义域错误
double sqrt (double x); x的平方根，其中x=0
double log (double x); 以e为底的对数,自然对数，x0
double log10 (double x); 以10为底的对数，x0
double ceil (double x); 取上整
double floor (double x); 取下整
double fabs (double x); x的绝对值
double frexp (double x, int *exp); 标准化浮点数, x = f * 2^exp, 已知x求f, exp ( x介于[0.5, 1] )并返回f值
double ldexp (double x, int exp); 与frexp相反, 已知x, exp求x*2^exp
double modf (double x, double *ip); 将参数的整数部分通过指针回传, 返回小数部分，整数部分保存在*ip中
double fmod (double x, double y); 返回两参数相除x/y的余数,符号与x相同。如果y为0，则结果与具体的额实现有关
包含头文件math.h，写成sin(x);的形式直接调用这个函数。其中x是double型弧度值。函数返回一个double值。如有double
x=3.1415926/180*30;，那么printf("sin30°
=
%f\n",sin(x));将输出sin30°
=
0.500000。
修改建议
您的回答内容不完整
知识型提问需要先直接对提问进行回复，开门见山，再对相关知识点进行延伸，如：加油哦！
你拒绝认证吧，这没有完善的必要了，学过三角函数的人都能看懂，看了10页C教科书的也都知道怎么写了。还要怎样“开门见山”，怎样“延伸”？难道还得解释sin是对边比斜边？
abs() 好像是stdlib.h的函数吧？
math.h
The math header defines several mathematic functions.
Macros:
HUGE_VAL
Functions:
acos();
asin();
atan();
atan2();
ceil();
cos();
cosh();
exp();
fabs();
floor();
fmod();
frexp();
ldexp();
log();
log10();
modf();
pow();
sin();
sinh();
sqrt();
tan();
tanh();
2.7.1 Error Conditions
All math.h functions handle errors similarly.
In the case that the argument passed to the function exceeds the range of that function, then the variable errno is set to EDOM. The value that the function returns is implementation specific.
In the case that the value being returned is too large to be represented in a double, then the function returns the macro HUGE_VAL, and sets the variable errno to ERANGE to represent an overflow. If the value is too small to be represented in a double, then the function returns zero. In this case whether or not errno is set to ERANGE is implementation specific.
errno, EDOM, and ERANGE are defined in the errno.h header.
Note that in all cases when it is stated that there is no range limit, it is implied that the value is limited by the minimum and maximum values of type double.
2.7.2 Trigonometric Functions
2.7.2.1 acos
Declaration:
double acos(double x);
Returns the arc cosine of x in radians.
Range:
The value x must be within the range of -1 to +1 (inclusive). The returned value is in the range of 0 to pi (inclusive).
2.7.2.2 asin
Declaration:
double asin(double x);
Returns the arc sine of x in radians.
Range:
The value of x must be within the range of -1 to +1 (inclusive). The returned value is in the range of -p/2 to +p/2 (inclusive).
2.7.2.3 atan
Declaration:
double atan(double x);
Returns the arc tangent of x in radians.
Range:
The value of x has no range. The returned value is in the range of -p/2 to +p/2 (inclusive).
2.7.2.4 atan2
Declaration:
double atan2(doubly y, double x);
Returns the arc tangent in radians of y/x based on the signs of both values to determine the correct quadrant.
Range:
Both y and x cannot be zero. The returned value is in the range of -p/2 to +p/2 (inclusive).
2.7.2.5 cos
Declaration:
double cos(double x);
Returns the cosine of a radian angle x.
Range:
The value of x has no range. The returned value is in the range of -1 to +1 (inclusive).
2.7.2.6 cosh
Declaration:
double cosh(double x);
Returns the hyperbolic cosine of x.
Range:
There is no range limit on the argument or return value.
2.7.2.7 sin
Declaration:
double sin(double x);
Returns the sine of a radian angle x.
Range:
The value of x has no range. The returned value is in the range of -1 to +1 (inclusive).
2.7.2.8 sinh
Declaration:
double sinh(double x);
Returns the hyperbolic sine of x.
Range:
There is no range limit on the argument or return value.
2.7.2.9 tan
Declaration:
double tan(double x);
Returns the tangent of a radian angle x.
Range:
There is no range limit on the argument or return value.
2.7.2.10 tanh
Declaration:
double tanh(double x);
Returns the hyperbolic tangent of x.
Range:
The value of x has no range. The returned value is in the range of -1 to +1 (inclusive).
2.7.3 Exponential, Logarithmic, and Power Functions
2.7.3.1 exp
Declaration:
double exp(double x);
Returns the value of e raised to the xth power.
Range:
There is no range limit on the argument or return value.
2.7.3.2 frexp
Declaration:
double frexp(double x, int *exponent);
The floating-point number x is broken up into a mantissa and exponent.
The returned value is the mantissa and the integer pointed to by exponent is the exponent. The resultant value is x=mantissa * 2^exponent.
Range:
The mantissa is in the range of .5 (inclusive) to 1 (exclusive).
2.7.3.3 ldexp
Declaration:
double ldexp(double x, int exponent);
Returns x multiplied by 2 raised to the power of exponent.
x*2^exponent
Range:
There is no range limit on the argument or return value.
2.7.3.4 log
Declaration:
double log(double x);
Returns the natural logarithm (base-e logarithm) of x.
Range:
There is no range limit on the argument or return value.
2.7.3.5 log10
Declaration:
double log10(double x);
Returns the common logarithm (base-10 logarithm) of x.
Range:
There is no range limit on the argument or return value.
2.7.3.6 modf
Declaration:
double modf(double x, double *integer);
Breaks the floating-point number x into integer and fraction components.
The returned value is the fraction component (part after the decimal), and sets integer to the integer component.
Range:
There is no range limit on the argument or return value.
2.7.3.7 pow
Declaration:
double pow(double x, double y);
Returns x raised to the power of y.
Range:
x cannot be negative if y is a fractional value. x cannot be zero if y is less than or equal to zero.
2.7.3.8 sqrt
Declaration:
double sqrt(double x);
Returns the square root of x.
Range:
The argument cannot be negative. The returned value is always positive.
2.7.4 Other Math Functions
2.7.4.1 ceil
Declaration:
double ceil(double x);
Returns the smallest integer value greater than or equal to x.
Range:
There is no range limit on the argument or return value.
2.7.4.2 fabs
Declaration:
double fabs(double x);
Returns the absolute value of x (a negative value becomes positive, positive value is unchanged).
Range:
There is no range limit on the argument. The return value is always positive.
2.7.4.3 floor
Declaration:
double floor(double x);
Returns the largest integer value less than or equal to x.
Range:
There is no range limit on the argument or return value.
2.7.4.4 fmod
Declaration:
double fmod(double x, double y);
Returns the remainder of x divided by y.
Range:
There is no range limit on the return value. If y is zero, then either a range error will occur or the function will return zero (implementation-defined).