Show that the ratio of the maximum tangential stress to the average tangential stress ( ) for a thick-walled cylinder subjected only to internal

Show that the ratio of the maximum tangential stress to the average tangential stress ( η ) for a thick-walled cylinder subjected only to internal pressure is (1+λ2 ) / (1+λ), where λ = Ro/Ri . Plot in a diagram the value of η as function of λ.