William W. answered • 09/25/19
Experienced Tutor and Retired Engineer
You should have written cos(theta) = -0.27735 but that was probably just a typo. However an angle with a negative cosine can either then be in quadrant 2 or it can be in quadrant 3 so it can either be 106 degrees or -106 degrees (which is 254 degrees).
But based on what I can see this looks generally correct. I don't know what the context is but I'm going to give you a BIG warning. Typically, when using formulas, you want to use radian measure instead of angle measure. It may not make any difference here but it totally depends on the context. Just be careful.
William W.
Well, hmmm. I know that the Excel function =DEGREES(xxxx) converts an angle in radians into an angle in degrees (so the result of =DEGREES(xxxx) is an angle in degrees and to give me that, "xxxxx" must be an angle in radians. HOWEVER, taking the cos of something does NOT give you an angle, it gives you a number between -1 and +1. So, the equation MUST be cos^-1 (cosine inverse) aka arccosine or in Excel lingo, its ACOS. So I suspect your equation should say =DEGRESS(ACOS(xxxx))09/25/19
RD D.
That was what we thought, just getting a second opinion. Thank you so much.09/25/19
RD D.
Thank you so much William. I am going to rephrase my question a bit. We are determining the angle of a pendulum test using a specific formula. The test standard says the equation we are to use is cos θ = 1-((M1+M2)/377. θ is the angle of the swing. M1=272, M2=209.56. We were given an excel formula as follows, but comes up with a smaller number for the degrees. =DEGREES(COS(1-(M1+M2)/377)) Thoughts? 55 degrees instead of 106 degrees.09/25/19