[" ore the "3times3times3times3times3times" identity "],[qquad [" 12.The value of the sum "sum_(i=1)^(20)i[(1)/(i)+(1)/(i+1)+(1)/(i+2)+cdots+(1)/(20)]" is "],[[" (1) "100," (2) "105],[" (3) "110," (4) "115]]]