.
It means that they talk about the sum of this geometric progression
a*r + a*r ^3 + a*r^5 + a*r^7 + . . .
which is taken over all odd powers of "r", and this sum does converge to 3.
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Dear "students" !
Today is a special day.
Today I solved 16 problems for you and got back 0 (zero, ZERO) "thanks" from you.
It is the absolute record !
It is just out of limits of my understanding !
I never saw it before !
Thank you very much and wish you to think: WHO ARE YOU . . .