Why does the classical law of addition of velocities contradict the special theory of relativity?

According to the classical law of addition of velocities, the speed of a body relative to a stationary frame of reference is equal to the sum of the speed of a body in a moving frame of reference and the speed of a moving frame of reference relative to a stationary one. However, it has been experimentally proven that the speed of light does not depend on the speed of the frame of reference, but is a constant value, i.e. invariant.

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