Trigonometric calculation not working

Question

My calculations show 75 * Cos(90) as zero. However, I get -22.46325...

I can confirm that it is NOT in radians because the radians value is -33.6055...

Here is what I have:

private static double RadianToDegree(double angle)
{
    return angle * (180.0 / Math.PI);
}

static void Main(string[] args)
{
    Console.WriteLine((75 * Math.Cos(RadianToDegree(90))).ToString());
}

Why am I getting a value of -22.46325 instead of 0?

SharpDX library has the MathUtil class that really helps with this sort of thing and it an MIT license — RadioSpace, May 02 '15 at 00:50

score 2 · Answer 1 · answered May 02 '15 at 00:00

2

input is taken as radians so you need to convert degrees to radians

answered May 02 '15 at 00:00

RadioSpace

953
7
15

score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

Math.Cos expects inputs in radians, not degrees:

public static double Cos(double d)

Parameters

d

An angle, measured in radians.

You said

My calculations show 75 * Cos(90) as zero

so presumably you are working in degrees.

You therefore need to use

private static double DegreeToRadian(double angle)
{
    return angle / (180.0 / Math.PI);
}

instead of

private static double RadianToDegree(double angle)
{
    return angle * (180.0 / Math.PI);
}

to convert the angle into radians as Math.Cos expects.

See this link to an ideone.com snippet to confirm it.

The result is 4.59227382683391E-15 but since you're dealing with floating point numbers, the result of Math.Cos(x) will never be zero (see, e.g. this StackOverflow question).

Just in case anyone is wondering how to get the value of zero or the whole numbers displayed on the calculator from this solution, cast the value of Math.Cos(x) as (int). Thanks everyone for your answers! — henrydavc, May 02 '15 at 03:37

score 0 · Answer 3 · answered May 02 '15 at 00:03

0

You're passing in an argument to Math.Cos that your code suggests should be in degrees, while Math.Cos takes Radians.

See: https://msdn.microsoft.com/en-us/library/system.math.cos%28v=vs.110%29.aspx

answered May 02 '15 at 00:03

meklarian

6,595
1
23
49

score 0 · Answer 4 · answered May 02 '15 at 00:05

0

You've got the conversion backwards:

private static double DegreesToRadians(double angle)
{

    return angle * (Math.PI / 180.0 );
}

static void Main(string[] args)
{
    Console.WriteLine((75 * Math.Cos(DegreeToRadians(90))).ToString());
}

answered May 02 '15 at 00:05

Richard Schwartz

14,463
2
23
41

score 0 · Answer 5 · answered May 02 '15 at 00:19

Your RadianToDegree method given the argument of 90 returns value of 5156.62015617741. Which is passed to Math.Cos. Math.Cos expects the angle in radians as an argument. That 5156.62015617741 radians in degrees would be:

5156.62015617741 / PI * 180 = 295452.571501057 = 252.571501057

cos of 252.571501057 degrees is -0.299515394, times 75 in your code gives -22.463654606 that you see (plus-minus floating point error).

As others already mentioned, you got your transformation backwards.

Trigonometric calculation not working

5 Answers5