1/3+1/15+1/35+...+1/575
=1/1*3+1/3*5+1/5*7+…+1/23*25
=1/2(1/1-1/3+1/3-1/5+…+1/23-1/25)
=1/2(1/1-1/25)
=1/2(24/25)
=12/25