The wave function of a molecular orbital formed by reinforce of wave functions of ${\mathrm{\Psi }}_{\mathrm{A}}$ and ${\mathrm{\Psi }}_{\mathrm{B}}$ of atomic orbital A and B is represented as

An atomic orbital is an electron wave, the waves of the two atomic orbitals may be in phase
or out of phase. Suppose ${\Psi }_A$ and ${\Psi }_B$ represent the amplitude of the electron wave of the
atomic orbitals of the two atoms A and B.
Case 1: When the two waves are in phase so that they add up and amplitude of the wave is $\mathrm{\Phi }={\Psi }_A+{\Psi }_B$