Transformers circuit question,where is my mistake?
This sounds like a school question.  So I'm going to give only hints.

1) what is the RMS voltage?

2) Assuming that the voltage source is perfect (zero source impedance), what does that voltage source look like at the secondary winding of the transformer.  In other words, you can easily calculate both the voltage and the effective source impedance given the information in your diagram.

3) Is the voltage at the secondary of the transformer in phase with the primary?

4) Use all of the above information and then add in the effect of the 8 Ohm resistor.