20. what is the 7th harmonic frequency of this ac sine wave?\na 10 hz\nb 14 hz\nc 21 hz\nd 42 hz

Explanation:

Step1: Calculate fundamental frequency

In 1 - second, there are 6 complete cycles. So the fundamental frequency $f_0=\frac{6}{1}=6$ Hz.

Step2: Calculate 7th harmonic frequency

The $n$th harmonic frequency $f_n = n\times f_0$. For $n = 7$ and $f_0=6$ Hz, $f_7=7\times6 = 42$ Hz.

Answer:

d. 42 Hz