｜1/21-1/20｜+｜1/22-1/21｜+｜1/23-1/22｜+...+ ｜1/30-1/29｜= (1/20 - 1/21) + (1/21 - 1/22) + (1/22 - 1/23) + ...+ (1/29 - 1/30)= 1/20 - 1/21 + 1/21 - 1/22 + 1/22 - 1/23 + ...+ 1/29 - 1/30= 1/20 - 1/3...