(1 / 21 + 1 / 31 + 1 / 41) * (1 / 31 + 1 / 41 + 1 / 51) - (1 / 21 + 1 / 31 + 1 / 41 + 1 / 51) * (1 / 31 + 41) 1 / 2)

(1 / 21 + 1 / 31 + 1 / 41) * (1 / 31 + 1 / 41 + 1 / 51) - (1 / 21 + 1 / 31 + 1 / 41 + 1 / 51) * (1 / 31 + 41) 1 / 2)

Simple calculation:
(1 / 21 + 1 / 31 + 1 / 41) * (1 / 31 + 1 / 41 + 1 / 51) - (1 / 21 + 1 / 31 + 1 / 41 + 1 / 51) * (1 / 31 + 1 / 41)
Suppose a = 1 / 31 + 1 / 41
The original formula = (1 / 21 + a) × (a + 1 / 51) - (1 / 21 + A + 1 / 51) × a
=1/21(a+1/51)+a(a+1/51)-1/21a-a(a+1/51)
=1/21(a+1/51)-1/21a
=1/21a+1/1071-1/21a
=1/1071
That is to use the substitution method to treat 1 / 31 + 1 / 41 as a number a
Substitution calculation, relatively simple!