The value of ( tan ⁡ β 2 + tan ⁡ α 1 ) {\displaystyle (\tan \beta _{2}+\tan \alpha _{1})\,} doesn't change for a wide range of operating points till stalling. Also α 1 = α 3 {\displaystyle \alpha _{1}=\alpha _{3}\,} because of minor change in air angle at rotor and stator, where α 3 {\displaystyle \alpha _{3}\,} is diffuser blade angle.