Posted by **Mimi** on Tuesday, August 24, 2010 at 3:40am.

Given that tan 45=1, use tan(x+y) to show that tan 22.5= /2 - 1.

(/is square root sign

- trig -
**MathMate**, Tuesday, August 24, 2010 at 8:27am
tan(x+y)=(tan(x)+tan(y))/(1-tan(x)tan(y))

substitute x=y to get

tan(2x) = 2tan(x)/(1-tan²(x))

Substitute tan(2x)=1 and solve for tan(x).

Reject the negative root knowing that tan(x)≥0 for 0<x<π/2.

- trig -
**Reiny**, Tuesday, August 24, 2010 at 8:28am
tan(x+y) = (tanx + tany)/(1 - tanxtany)

so tan 45

= tan(22.5 + tan22.5)/(1 - tan22.5tan22.5)

= 2tan22.5)/1 - tan^2 22.5)

let tan22.5 = y for ease of typing

1 = 2y/(1 - y^2)

1 - y^2 = 2y

y^2 + 2y - 1 = 0

y^2 + 2y = 1 , by completing the square

y^2 + 2y + 1 = 1+1

(y+1)^2 = 2

y+1 = √2

y = √2-1

tan22.5° = √2 - 1

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