(1 - 2/6) × (1 - 2/12) × (1 - 2/20) × ... × (1 - 2/9900)
= 4/6 × 10/12 × 18/20 × ... × 9898/9900
= 1.4/2.3 × 2.5/3.4 × 3.6/4.5 × ... × 98.101/99.100
= 1.2.3...98/3.4.5...100 × 4.5.6...101/2.3.4...99
= 2/99.100 × 100.101/2.3
= 101/99×3 = 101/297
☆☆☆☆☆