a+b=4/7
ab=3/4
1/2a^3b+1/2ab^3
=1/2ab(a^2+b^2)
=1/2ab[(a+b^2)-2ab]
=1/2 * (ab) * (a+b)^2 - (ab)^2
=1/2 * 3/4 * (4/7)^2 - (3/4)^2
=2/49 - 9/16
=-409/784