Prove Theorem 6.33(b).

Let S be the solution space of

y'' + ay' + by = 0
and let λ₁ and λ₂ be the roots of the characteristic equation λ² + aλ +  b = 0.

b. If λ₁ = λ₂, {e^λ1t, e^λ2t}then is a basis for S.