请详细解释1/5x9+1/9x13+1/13x17+......
1/5x9+1/9x13+1/13x17+......+1/101x105=?
1/5x9+1/9x13+1/13x17+......+1/101x105 =1/4(1/5-1/9)+1/4(1/9-1/13)+1/4(1/13-1/17)+......+1/4(1/101-1/105) =1/4(1/5-1/105) =1/21.