Look at my simulation where it shows a positive peak of +5.1V and a negative peak of -4.4V. They are different due to distortion. Their total is 9.5V.Did you mean that your sinewave has 2 peaks- Positive 5.1V and Negative -4.4V? So due to this two peak voltages you multiplied 1.414 x 2? But all AC has negative and positive peaks......um....
Can you say in basic please!
Each of the two peaks has an RMS voltage that is the peak divided by 1.414 so both have an average RMS voltage that is the total divided by 2.828.