In amplitude modulation, if the amplitude of the message signal is increased by \(20\%\) and the amplitude of the carrier wave is decreased by \(20\%\), then the percentage increase in the modulation index is
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The modulation index in AM is
\[
\boxed{
m=\frac{A_m}{A_c}.
}
\]
Any percentage change in \(A_m\) or \(A_c\) directly changes the modulation index according to this ratio.