Question

how two add two signal with two different frequency

Answer 1

Consider this
\[A \cos 2\pi f1+A \cos 2\pi f2\]Summing or superimposing them gives the result as \[2A \cos 2\pi \frac{ f1-f2 }{ 2 }\cos 2\pi \frac{ f1+f2 }{ 2 }\]This is a result of simple trigonometric manipulation.