ab=1,
1/(a²+1)+1/(b²+1)
=(b²+1)/(a²+1)(b²+1)+(a²+1)/(b²+1)(a²+1)
=(a²+1+1+b²+1)/(a²+1)(b²+1)
=(a²+2+b²)/(a²b²+a²+b²+1)
=(a²+2+b²)/(1+a²+b²+1)
=1