The wavefunction ψ(x) = ϕ1(x) + 2ϕ2(x) + 3ϕ3(x) is to be normalised. The functions ϕ1(x), ϕ2(x), ϕ3(x) are normalised eigenfunctions of a Hermitian operator ˆO with eigenvalues λ1=1, λ2=5, λ3=9. How can we integrate the long sum of the combination of the ϕ(x)s to normalise the wavefunction?