F(-∞)A+(1/π)arctan(-∞)=A-π/2π=0 A=1/2 D(AX+B)=A^2D(X) D（3X-6）=9D(X)=4 D(X)=9/4 EX=np=12 DX=np(1-p)=8 1-p=3/4 p=1/4