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tan( 2Π/9 ) = cot( 13Π/18 )

  1. Draw angles

  2. This is case B.

  3. Proof

There is a 90degree disparity between angle

2Π/9

and angle

13Π/18

Angle

is the bigger one of the two

13Π/18

So,

13Π/18

2Π/9

Should equal Π/2

L.S

R.S

13Π/18

2Π/9

Π/2

13Π/18

4Π/18

9Π/18

Π/2

L.S = R.S!!!!!!

cot(Π/2 - 2Π/9) = tan(2Π/9)