In Problems 23-30 verify that the given functions form a fundamental set of solutions of the differential equation on the indicated interval. Form the general solution.
23. y'' - y' - 12y = 0; e^-3x, e^4x, (-∞, ∞)
24. y'' - 4y = 0; cosh 2x, sinh 2x, (-∞, ∞)
25. y'' - 2y' + 5y = 0; e^x cos 2x, e^x sin 2x, (-∞, ∞)
26. 4y'' - 4y' + y = 0; e^x/2, xe^x/2, (-∞, ∞)
27. x^2y'' - 6xy' + 12y = 0; x^3, x^4, (0, ∞)
28. x^2y'' + xy' + y = 0; cos(ln x), sin(ln x), (0, ∞)
29. x^3y''' + 6x^2y'' + 4xy' - 4y = 0; x, x^-2, x^-2 ln x, (0, ∞)
30. y^(4) + y'' = 0; 1, x, cos x, sin x, (-∞, ∞)