2．计算：

(1) 2/3 + 1/3 + 1/5 + 2/15 + 2/21 + 1/14 + 1/18 = (2/3 + 1/3) + (1/5 + 2/15) + (2/21 + 1/14) + 1/18 = 1+ 1/3 + 1/6 +1/18 = 14/9(2) 1/n(n+1)(n+2)(n+3)=1/3*[1/n(n+1)(n+2)-1/(n+1)(n+2)(n+3)]1/1*2*3*4+1/2*3*4*5+。+1/7*8*9*10=1/3*(1/1*2*3-1/2*3*4+1/2*3*4-1/3*4*5+。+1/7*8*9-1/8*9*10)=1/3*(1/1*2*3-1/8*9*10)=119/2160