An=4/(n+1)(n+3)數列求和

An=4/(n+1)(n+3)數列求和

用列項相消,4/(n+1)(n+3)=2(1/n+1-1/n+3),則Sn=2(1/2+1/3-1/n+2-1/n+3)=5/3-4n+10/(n+2)(n+3),