概華论与款醒硫外 7a2=502.92-10 ×1.645=498.17, W12 na12=502.92+.10 ×1.645=507.67, W12 即μ的置信度为90%的置信区间为 (498.17,507.67). (2)当a=0.05时，1-0=0.975， +  / 2 =  z n x 1.645 12 10 502.92 +  = 507.67, −  / 2 =  z n x 1.645 12 10 502.92 −  = 498.17, 即 的置信度为90%的置信区间为 (498.17, 507.67). (2)当 = 0.05时, 0.975, 2 1− = 