The sum of infinite terms of the progression 1+3x+5x2+7x3+.......(|x|<1) ?
Sn = 1 + 3x + 5x² + 7x³ +
xSn = x + 3x² + 5x³ +
Sn(1-x) = 1 + 2x + 2x² + 2x³
Sn(1-x) = 1 + 2x(1 + x + x² + x² +
Sn(1-x) = 1 + 2x(1/(1-x))
Sn(1-x) = (1+x)/(1-x)
Sn = (1+x)/(1-x)²