Question

I am working on a simple inverse kinematic delta robot controller with Processing. I am stuck at the rule of cosines. I have the length of the three sides and wish to get the angles. But float angle = acos((sq(humerus)+sq(ulna) - sq(radius))/(2 * humerus * ulna));
always returns NaN. Any ideas?

Answer 1

You’ll probably want to use the law of cosines if you know the sides:

In Java terms, to solve for the angle C (opposite to side of length c) we would have

Math.acos((a*a + b*b - c*c) / (2*a*b))

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There are a couple reasons why you could be getting NaN:

• One of your side lengths is negative, so when you square root it you receive NaN.
• Your triangle can’t really exist based on the side lengths you specified. Look at this documentation for acos:
  

  If the argument is NaN or its absolute value is greater than 1, then the result is NaN.