Conan Edogawa 837 Posted January 31, 2016 Report Share Posted January 31, 2016 Ok so many people might be looking at me and saying "what"?? As follows, the theorems are these. sin 2a = 2sina cosa cos 2a = cos2a - sin2a = 1 - 2sin2a = 2cos2a - 1 Now some of you might be saying "how will this help me make TRIG EASIER????" Well it already does! Question 1. Find cos(22.5) Well 22.5=45/2 thus and then cosine(22.5) would then equal 0.923, using mental math. QED! 1 Quote Link to post Share on other sites
Shawnic 2,977 Posted January 31, 2016 Report Share Posted January 31, 2016 2 minutes ago, Chain Rule said: and then cosine(22.5) would then equal 0.923, using mental math. Very funny. The entire human race has a trig calculator built in their brains. Quote Link to post Share on other sites
Conan Edogawa 837 Posted January 31, 2016 Author Report Share Posted January 31, 2016 Just now, Master Flap said: Very funny. The entire human race has a trig calculator built in their brains. It seems that Master Flap has not found this. Quote Link to post Share on other sites
Shawnic 2,977 Posted January 31, 2016 Report Share Posted January 31, 2016 1 minute ago, Chain Rule said: It seems that Master Flap has not found this. How would you evaluate sqrt(1 + sqrt(2)/2) mentally? Quote Link to post Share on other sites
Conan Edogawa 837 Posted January 31, 2016 Author Report Share Posted January 31, 2016 7 hours ago, Master Flap said: How would you evaluate sqrt(1 + sqrt(2)/2) mentally? 1+sqrt(2)=2.141 approx. so divide that by 2 and you get 1.07. Square root of that is approx .98 something. Quote Link to post Share on other sites
Kanad 351 Posted February 1, 2016 Report Share Posted February 1, 2016 Well, I've done the Trignometry chapters a few times. Never seen those formulas before. Are they taught in schools or not? Quote Link to post Share on other sites
Conan Edogawa 837 Posted February 1, 2016 Author Report Share Posted February 1, 2016 6 hours ago, Kanad said: Well, I've done the Trignometry chapters a few times. Never seen those formulas before. Are they taught in schools or not? Yes, they are. Quote Link to post Share on other sites
Shawnic 2,977 Posted February 1, 2016 Report Share Posted February 1, 2016 18 hours ago, Chain Rule said: 1+sqrt(2)=2.141 approx. so divide that by 2 and you get 1.07. Square root of that is approx .98 something. How do you find square roots mentally? Quote Link to post Share on other sites
Joyeuse 246 Posted February 1, 2016 Report Share Posted February 1, 2016 16 hours ago, Kanad said: Well, I've done the Trignometry chapters a few times. Never seen those formulas before. Are they taught in schools or not? Yes and they're miserable to do. I was never good at them. Quote Link to post Share on other sites
Conan Edogawa 837 Posted February 1, 2016 Author Report Share Posted February 1, 2016 8 hours ago, Master Flap said: How do you find square roots mentally? -__- The square root of 27 is between 5 and 6... 1 Quote Link to post Share on other sites
Insider 290 Posted February 2, 2016 Report Share Posted February 2, 2016 You guys are scaring me... 1 Quote Link to post Share on other sites
Serpent-Z 84 Posted February 2, 2016 Report Share Posted February 2, 2016 Well there are also triple angle formulae, and half angle formulae for triangle angles sinA/2 = sqrt((s-b)(s-c)/bc) and cosA/2 = sqrt(s(s-a)/bc) Quote Link to post Share on other sites
Conan Edogawa 837 Posted February 2, 2016 Author Report Share Posted February 2, 2016 6 hours ago, Serpent a.k.a. Zenith said: Well there are also triple angle formulae, and half angle formulae for triangle angles sinA/2 = sqrt((s-b)(s-c)/bc) and cosA/2 = sqrt(s(s-a)/bc) Yes, but that is a offshoot of the two original theorems above. Quote Link to post Share on other sites
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