# Prove t = x*y => t*x = x*t^(-1)

xsw001
Let x, y be elements of order 2 in any group G.
Prove that if t = xy, then t*x = x*t^(-1)

Here is what I got so far.
Proof:
Since |x| = 2 => x^2 = 1; |y| = 2 => y^2 = 1, then (x^2)(y^2) = 1 => (xy)^2 = 1
Suppose t = xy, then t^2 = (xy)^2 = 1
WTS (want to show) t*x = x*t^(-1)

This group looks like Dihedral group D2n, the operation t*x = x*t^(-1) similar to r*s = s*r^(-1),
we know r^n = s^2 = 1 in D2n, since x^2 = 1, we can let x = s, so x^2 = s^2 = 1. Hence we've shown x = s,
Now I need to show t^n = (xy)^n = 1 so that I can let t = xy = r,
and if we show t = xy = r, then we get exactly the same operation in D2n such that r*s = s*r^(-1) where r = t = xy and s = x

Here is what I stuck, how do I show t^n=(xy)^n=1 ? I only know t^2 = (xy)^2 = 1 from the given condition. Now I need to derive from there and to show that t^n = (xy)^n

Any suggestions?

BTW, I think that I post in the wrong session. I'll make a note next time since I can't delete the post.

Homework Helper
Gold Member
Notice that $$t^{-1}=y*x$$ because $$t*(y*x)=(t*y)*x=(x*(y*y))*x=(x*1)*x=1$$and similarly $$(y*x)*t=1$$.

xsw001
Yes! Got it!
Since t^-1=(y^-1)*(x^-1) and x^2=1 => x=x^-1, y^2=1 => y=y^-1, so t^-1=yx
Then tx=xyx, and x(t^-1)=xyx, indeed they are equal!
Thanks for the tips Delta! Now it seems so simple!

Last edited:
Homework Helper
Gold Member