有關統計問題~急需知道答案?

1a)
E(X)=500
Var(X)=50^2=2500

1b
X~(500,50^2)
P(X<600)
=P(Z<2)
=0.97725

1c
P(X>400)
=P(Z>-2)
=1-0.02275
=0.97725

1d
P(450<X<600)
=P(-1<Z<2)
=0.47725+0.34134
=0.81859

2a
是，因為n=30 or n>30

2b
X~(28000,5000^2)
P(X>30000)
=P(Z>0.4)
=0.5-0.15542

3a
X~(30,5^2)
P(25<X<40)
=P(-1<X<2)
=0.34134+0.47725
=0.81859

3b
P(X<38)
=P(Z<1.6)
=0.9452

4a
X~(75,10^2)
P(X<70)
=P(Z<-0.5)
=0.30854

4b
P(60<X<80)
=P(-1.5<Z<0.5)
=0.43319+0.19146
=0.62465

4c
P(X<70)=0.30854
P(60<X<80)=0.62465

5a
X~(50-48,(5-6)^2)
X~(2,1^2)
P(X1-X2<4)
=P(Z<2)
=P0.97725

5b
P(X1-X2>5)
=P(Z>3)
=0.5-0.49865
=0.00135

5c
P(3.5<X1-X2<5)
=P(1.5<Z<3)
=0.49865-0.43319
=0.06546