i have a triangle. side a=3. side b=4. side c=2.1 ( this one is on the opposite side of theta ). the formula is cos theta = ( (a*a) + (b*b) - (c*c) ) / (2ab) i got : cos theta = 0.8579166667 now how to get the angle of theta?? i tried to do arc cos using my calculator, but the result is 0.5395954398 i KNOW that the angle of theta should be 30.05 degree ( according to my protractor ). how to get 30.05 out of that calculation? is my calculator calculating the numbers in radian? btw this is for a comp graphics class

Yeah, it looks like the resulting answer is in rads. I just checked what 30.05 in rads was, it was .52447144, pretty similar to what you got on the second answer... EDIT: Okay, i got it... I think, its not exact though. The answer was 30.92 the formula i used was c^2=(a^2)+(b^2)-2(a)(c)cosC (i htink your formula is the same, just written differently. I came up with: 2.1^2=(4^2)+(3^2)-2(4)(3)cosC 2(4)(3)cosC = (16)+(9)-(4.41) 24(cosC) = 20.59 cosC = 20.59/24 then i used cos^-1 on my calculator, and found 30.91653504

Well on my calculator, if you're in Degree mode, just type n the degree, and when you swithc to rads, itll display the degrees in Rads (mine's a SHARP). But to do it the long way, you take the degrees, multiply by Pi, and then divide by 180: [c]30.92(Pi) 180[/c] Equals .539655804 rads

Or you could do it the hard way and use a Taylor series to approximate acos (which is all the calculator is doing, anyway). I'd recommend you forget degrees exist and learn to use radians almost exclusively. Most graphics (and physics) equations that deal with angles will require radians.