∫(－∞,＋∞）f(x)dx
=∫[0,1]axdx
=ax^2/2[0,1]
=a/2=1
a=2
P（X≥1/2）
=∫[1/2,1]2xdx
=x^2[1/2,1]
=3/4
F(x)=∫(－∞,＋∞）f(x)dx
=0 (x1)