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11.(x-1)^5=[(x+1)-2]^5,
所以a2=C(5,3)*(-2)^3=-80.选D.
15.作变换t=(X-10)/σ,则
P(X>12)=Φ(t>2/σ)=m,
P(8<X<10)=Φ(-2/σ<t<0)=0.5-m=n,
所以2/m+1/n=2/m+1/(0.5-m)=w,
w'=-2/m^2+1/(0.5-m)^2=0,
2(0.5-m)^2=m^2,
m^2-2m+0.5=0,0<m<0.5,
解得m=1-√0.5,此时w=2/(1-√0.5)+1/(√0.5-0.5)
=4(1+√0.5)+4(√0.5+0.5)
=4(1.5+2√0.5)
=6+4√2.
18.(1)由A,各加1,3<=3x<=9,
各除以3,得1<=x<=3,
由B,2x<6,x<3,
所以A∩B=[1,3),
A∪B=(-∞,3].
(2)C的补集=(a,a+1)是A的子集,
等价于1<=a,且a+1<=3,
解得1<=a<=2.
所以a2=C(5,3)*(-2)^3=-80.选D.
15.作变换t=(X-10)/σ,则
P(X>12)=Φ(t>2/σ)=m,
P(8<X<10)=Φ(-2/σ<t<0)=0.5-m=n,
所以2/m+1/n=2/m+1/(0.5-m)=w,
w'=-2/m^2+1/(0.5-m)^2=0,
2(0.5-m)^2=m^2,
m^2-2m+0.5=0,0<m<0.5,
解得m=1-√0.5,此时w=2/(1-√0.5)+1/(√0.5-0.5)
=4(1+√0.5)+4(√0.5+0.5)
=4(1.5+2√0.5)
=6+4√2.
18.(1)由A,各加1,3<=3x<=9,
各除以3,得1<=x<=3,
由B,2x<6,x<3,
所以A∩B=[1,3),
A∪B=(-∞,3].
(2)C的补集=(a,a+1)是A的子集,
等价于1<=a,且a+1<=3,
解得1<=a<=2.
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2020-04-07
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第一题没改数据的话我答案都背下来了/笑哭
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