=(1/21+1/80)+(1/22+1/79)+...+(1/40+1/41)
=(101/21x80+101/21x80)+(101/22x79+101/22x79)+...+(101/40x41+101/40x41)
=101[(1/21x80+1/21x80)+(1/22x79+1/22x79)+...+(1/40x41+1/40x41)]
Vì (1/21x80+1/21x80)+(1/22x79+1/22x79)+...+(1/40x41+1/40x41) ko phải là số tự nhiên nên 1/21+1/22+1/23+...+1/80 ko phai số tự nhiên