What is the derivation formula of sinusoidal AC RMS? Such as the title

What is the derivation formula of sinusoidal AC RMS? Such as the title

Assuming that AC U (T) = asint and DC U (T) = B have the same ability to do work for resistance R, then B is the effective value of AC voltage asint. In the time of 0-2 π, the integral of AC power ∫ (asint) ^ 2DT / r = π a ^ 2 / R, while the work done by DC during this time is 2 π B ^ 2 / R. therefore, π a ^ 2 / r = 2 π B ^ 2 / R, a