3x5分之1+5x7分之1+7x9分之1+......+21x23分之一=1/(3X5)+1/(5X7)+1/(7X9)+.......+1/(21X23)=(1/2) (1/3-1/5+1/5-1/7+1/7-1/9+......+1/21-1/23)=(1/2) (1/3-1/23)=(1/2) (20/69)=10/69
