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Below are four wave equations describing a wave moving at a velocity of, $v=15 \text{ m/s}$. One of the wave equations is NOT possible.

Which wave is NOT possible?

$y(x,t)=3.5\cos (3.14t-0.209x)$

$y(x,t)=7.8\cos (9.42t-0.070x)$

$y(x,t)=1.5\cos (6.28t-0.419x)$

$y(x,t)=4.0\cos (1.57t-0.105x)$