原式=(1/5+1/7+1/9+1/11)*(1/7+1/9+1/11)+ (1/5+1/7+1/9+1/11)* 1/13 -(1/5+1/7+1/9+1/11+1/13)*(1/7+1/9+1/11)
=(1/5+1/7+1/9+1/11)* 1/13 + (1/5+1/7+1/9+1/11 - 1/5-1/7-1/9-1/11-1/13)*(1/7+1/9+1/11)
=(1/5+1/7+1/9+1/11)* 1/13 - 1/13*(1/7+1/9+1/11)
= 1/13 * (1/5+1/7+1/9+1/11 - 1/7-1/9-1/11)
= 1/13 * 1/5
= 1/65