hi I'm stuck on finding the second derivative of:

y = (1+x^2)/(1+2x^2)

By hand I get:

2(8x^(2) - 1)/(2x^(2) + 1)^2 after simplifying

However, an online calculator gives me:

2(6x^(2)- 1)/(2x^(2) + 1)^2

I've checked y' but keep on getting the same answer.

Am I wrong?

Thanks, D