If the waves are coherent, then the resultant intensity is given byI = I1 + I2 + 2 √(I1 I2 Cos Θ), where Θ is the phase difference between two waves. For the constructive interference the value of Θ = 0°, so that the Cos Θ = 1 and for the destructive interference the value of Θ = 90°, so that the Cos Θ = 0. In case of the incoherent sources or waves we can the intensity simply by algebraic method that means I = I1 + I2. Watch the video below for detailed derivation