# How can I calculate this sum? (accept both sage(cocalc) and by hand)

How can I calculate this sum?

1/1-(x+x^2)^2

- This is the sum of infinity formula : a/1-r, while a=1 and r=(x+x^2)^2 Please also confirm if my formula is right.

I'm afraid that your formula is

notcorrect :This can be shown in an elementary fashion by doing the "long division" of 1 by (1-r) "by hand"...

Could you restate your question a bit more clearly ? Xhat is summed, for what variable ?

What's your formula equal to???

My formula is equal to a/(1-r), for that is for adding a certain sequence to infinity. So now I have to apply to 1/1-(x+x^2)^2.

Do you mean "That" when you type "Xhat"? (to Emmanuel Charpentier)

Anyone could answer me after all?