The correct option is A) I and III, and also II and IV.
The equation of wave is given by;
y = Asin(kx - ωt)
or
y = Acos(kx - ωt)
(depending or the waveform)
where;
A is the amplitude of the wave in meters
y(x,t) is the displacement in meters
k = 2π/λ is the propagation constant
ω = 2π/T is the angular frequency in radian per seconds
T = 2π/ω;
T is the time period in seconds
+ve and -ve signs show the direction of the wave.
I) Comparing y = 0.12cos(3x - 21t) with the general equation;
ωt = 21t
ω = 21
2π/T = 21
T = 2π/21 seconds
II) Comparing y = 0.15 sin(6x + 42t) with the general equation;
ωt = 42t
ω = 42
2π/T = 42
T = 2π/42 seconds
III) Comparing y = 0.13 cos(6x + 21t) with the general equation;
ωt = 21t
ω = 21
2π/T = 21
T = 2π/21 seconds
IV) Comparing y = -0.23 sin(3x - 42t) with the general equation;
ωt = 42t
ω = 42
2π/T = 42
T = 2π/42 seconds
Therefore I and III have the same period, II and IV have the same period.
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