Shafts with variable diameter, shafts are in series

Shafts with variable diameter, shafts are in series, stepped shaft, multiple torque, angle of twist, equation of torsion,
How to calculate angle of twist of a stepped shaft (shafts are in series) and shaft with variable cross section.
Strength of materials, GATE Mechanical solutions.
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[@harshalnerikar4650]

In a uniformly varying shaft, how can you consider that the starting diameter is twice the end diameter 8:11

[@MechanicalAcademy]

That is a given parameter.