下今用钢尺丈量得两段距离：S1 = 120.63 ± 6.1cm, S2 =114.49 ± 7.3cm，试求距离S3 = S1 + S2 和S4 = S1 - S2 的中误差和它们的相对中误差。
答：S3 = S1 + S2 = 235.12m (1分)m3 = (m1* m1 + m2* m2 ) 1/2 = 9.5 cm (3分)
K3 = m3 / S3 = 1/2475 (5分)
S4 = S1 - S2 = 6.14m (6分)
m4 = (m1* m1 + m2* m2 ) 1/2 = 9.5 cm (8分)
K4 = m4 / S4 = 1/65 (10分)
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