wuzx · 2022年01月17日
NO.PZ2019040801000013
问题如下：
X's probability is shown as following,
P(X=0.4)=30%, P(X=0.2)= 30%, P(X=-0.2)= 40%.
the standard deviation of X is closest to:
选项：
A. 0.1257.
B. 0.2138.
C. 0.2569.
D. 0.1566.
解释：
C is correct
考点：Expected Value & Variance
解析：先求出X的期望 = 30%*(0.4) + 30%*(0.2) + 40%*(-0.2) = 0.1
那么X的方差就是 30%*(0.4-0.1)2+ 30%*(0.2-0.1)2 + 40%*(-0.2 -0.1)2 = 0.066
Standard deviation = 0.2569
两个问答看了还是不懂最后一步NO.PZ2019040801000013问题如下X's probability is shown following,P(X=0.4)=30%, P(X=0.2)= 30%, P(X=-0.2)= 40%.the stanrviation of X is closest to:A.0.1257.B.0.2138.C.0.2569.0.1566.C is correct考点ExpecteValue Variance解析先求出X的期望 = 30%*(0.4) + 30%*(0.2) + 40%*(-0.2) = 0.1那么X的方差就是 30%*(0.4-0.1)2+ 30%*(0.2-0.1)2 + 40%*(-0.2 -0.1)2 = 0.066Stanrviation = 0.2569E（x）是能求的，E（x平方）是否等于x平方乘以x概率的平方呢？
NO.PZ2019040801000013问题如下X's probability is shown following,P(X=0.4)=30%, P(X=0.2)= 30%, P(X=-0.2)= 40%.the stanrviation of X is closest to:A.0.1257.B.0.2138.C.0.2569.0.1566.C is correct考点ExpecteValue Variance解析先求出X的期望 = 30%*(0.4) + 30%*(0.2) + 40%*(-0.2) = 0.1那么X的方差就是 30%*(0.4-0.1)2+ 30%*(0.2-0.1)2 + 40%*(-0.2 -0.1)2 = 0.066Stanrviation = 0.2569E（x^2）不应该是x^2✖️概率再相加吗
NO.PZ2019040801000013问题如下 X's probability is shown following,P(X=0.4)=30%, P(X=0.2)= 30%, P(X=-0.2)= 40%.the stanrviation of X is closest to:A.0.1257.B.0.2138.C.0.2569.0.1566.C is correct考点ExpecteValue Variance解析先求出X的期望 = 30%*(0.4) + 30%*(0.2) + 40%*(-0.2) = 0.1那么X的方差就是 30%*(0.4-0.1)2+ 30%*(0.2-0.1)2 + 40%*(-0.2 -0.1)2 = 0.066Stanrviation = 0.25690这个答案怎么算出来的？求步骤
NO.PZ2019040801000013 P(0.4)=30%, P(0.2)=30%, P(-0.2) = 40% 在求出来E（x）之后求方差为什么前面的weights: w(0.4)=30%; w(0.2)和w(-0.2)没有平方？只平方了（X- E(x)）?
