1) 1/n(n+1) =1/n - 1/(n+1)
2)原式=1/1*2 - 1/2*3 - 1/3*4 - ...- 1/21*22
=1 - 1/2 - (1/2 - 1/3) - (1/3 - 1/4) - ...-(1/21 - 1/22)
=1/22
1) 1/n(n+1) =1/n - 1/(n+1)
2)原式=1/1*2 - 1/2*3 - 1/3*4 - ...- 1/21*22
=1 - 1/2 - (1/2 - 1/3) - (1/3 - 1/4) - ...-(1/21 - 1/22)
=1/22