Who can figure it out? I admire him! 7/12-9/20+11/30-13/42+15/56-17/72+19/90

Who can figure it out? I admire him! 7/12-9/20+11/30-13/42+15/56-17/72+19/90

The original formula = (1 / 3 + 1 / 4) - (1 / 4 + 1 / 5) + (1 / 5 + 1 / 6) - (1 / 6 + 1 / 7) + (1 / 7 + 1 / 8) - (1 / 8 + 1 / 9) + (1 / 9-1 / 10) = 1 / 3-1 / 10 = 7 / 30

Drag type is easy to calculate, give more points
(6 / 1 - 7 / 1) by 42 13 / 2 by 17 / 15 - 13 / 2 by 17 / 2 by 89 by 88 / 5

The numerator and denominator of your questions are reversed. I think it's the following fraction. I wonder if I'm right. (1 / 6-1 / 7) × 42 = 1 / 6 × 42-1 / 7 × 42 = 7-6 = 12 / 13 × 15 / 17-2 / 13 × 2 / 17 = 2 / 13 × (15 / 17-2 / 17) = 2 / 13 × 13 / 17 = 2 / 17 89 × 5 / 88 ...