X=1 P=0.4
X=2 P=0.6*0.4=0.24
X=3 P=0.6*0.6*0.4=0.144
····
X=n P=0.6^(n-1) *0.4
所以P（X=奇数）=P（X=1）+P（X=3）+···=0.4+(0.6^2)*0.4+(0.6^4)*0.4+···
=0.4（1+0.6^2+0.6^4+···）此处相当于等比数列无限项的和S=a1/（1-q）=1/（1-0.6^2）=25/16
=0.4*25/16
=5/8
所以X为奇数的概率为5/8