离散型随机变量分布律应满足两个条件；⑴随机变量取每个值的概率非负；⑵随机变量取每个值的概率之和为1.
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设两个相互独立的随机变量X和Y的方差分别为6和3，则D(2X-3Y)=
A. 51
B. 21
C. -3
D. 36
Question 1
A. Teaching English.
B. Selling books.
Cleaning the house.
D. Taking care of children
Question 2
A. Because the children she was taking care of were sick.
Because the children she was taking care of were badly treated.
C. Because she took the children to see an X-rated movie called Girls.
D. Because she took the children to see a R-rated movie called Girls.
Question 3
A. X-rated movies.
B. G movies.
C. R-rated movies.
D. PG 13 movies.
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