Can You Evaluate cos36 − cos72 😄

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[@armandodluca266]

Nicely done! Keep up the good work!

[@SyberMath]

Thanks, you too!

[@honestadministrator]

cos (36°) - cos (72°)
= 1 - 2 sin^2 ( 18°) - sin (18°)
= 1 - sin(18°) [ 2 sin (18°) - 1]
Here in 2 sin(18°) = ( √5 - 1) /2

[@scottleung9587]

Nice!

[@SyberMath]

Thanks!

[@phill3986]

😊😊😊👍👍👍

[@SweetSorrow777]

Aww. No radians? I didn't at first, but it's grown on me to the point that I don't bother with degrees unless I must convert between the 2.

[@comic4relief]

In a purely Euclidean sense, the basic unit can be the right angle.
2π/5 rad is four fifths of a right angle.
π/5 =2/5 right angle.
4/5 + 4/5 + 2/5 = 2 right angles.
In any triangle the sum of the angles is equal to two right angles.

[@terryendicott2939]

Take a regular pentagram with side of length 1. Have one vertex on the y-axis and the opposite side on the x-axis. Cut it in half and throw the quadrant II stuff away.. Now get the abscissa of point that is not on either axis. It will be 1/2 + cos 72 and also cos 36. Done.

[@SyberMath]

I’m not sure what you mean. Can you share a picture?

[@giuseppemalaguti435]

cos36-cos72=cos36-(2(cos36)^2-1).....cos36=(1+√5)/4 per costruzione (basta costruire un triangolo isoscele di angoli 36-36-108.......)..quindi..x=(1+√5)/4-2((1+√5)/4)^2+1=1/2

[@DonEnsley-yi2ql]

½

[@vladimirkaplun5774]

11 mins of wasted time and finally the canonical solution in 2 mins. Normal student do it without even applying brains. Spinal cord is enough

[@SyberMath]

😮😄😁😜

[@finnboltz]

First.