FactChecker said:
It might help to know that there are certain times when all the waves come back to their initial, t=0, values. For instance, at t=39,104 we have 39,104/13=3,008; 39,104/6.4=6,110; and 39,104/18.8=2080. So at that time, all the waves will start repeating. I believe this is the earliest time for them to all start repeating in unison.
I got this by factoring 130, 64, and 188 to get the Least Common Multiple (LCM), 195,520. Then I divided that by 10. At that time, all the angles are integer multiples of ##\pi##, but one is not a multiple of ##2\pi##. Multiplying the LCM by 2 gives a time, t=39,104 where all angles are integer multiples of ##2\pi##.
At this time, it is not immediately clear to me what to do with this fact. I believe it would explain why I can not find an amplitude of 1475 or higher on a graph.
Consider two periodic continuous (not necessarily sinusoidal) real functions of time, ##y_1## and ##y_2##, with respective periods ##T_1## and ##T_2##.
The sum, ##Y = y_1 + y_2##, has a period which is the least common multiple (LCM) of ##T_1## and ##T_2##. That’s the shortest time-interval both functions will have simultaneously completed an exact integer number cycles.
So to find the amplitude of ##Y##, it is necessary to find the maximum value during a time interval equal to LCM(##T_1, T_2)##.
The argument extends to the sum of any number of periodic functions.
_________
Now consider the original problem. Note that the function ##y = sin(\omega t)##, for example, has period ##T = \frac {2\pi}{\omega}##.
In the Post #1 function, there are 3 different angular frequencies: ##\omega_1 = 13\pi, \omega_2=6.4\pi## and ##\omega_3=18.8\pi##.
The corresponding 3 periods are therefore:
##T_1= \frac {2\pi}{13\pi} = 2/13##
##T_2= \frac {2\pi}{6.4\pi} = 2/6.4 = 20/64 = 5/16##
##T_3=\frac {2\pi}{18.8\pi} =2/18.8 = 20/188 = 5/47##
LCM(##T_1, T_2, T_3##) = LCM (2/13, 5/16, 5/47) = 10, so the amplitude corresponds to the largest value within any time-interval of 10 (unknown time-units).
Carefully inspecting a graph covering t=0 to t=10, I found the maximum is 1479.173 at t=4.78611, though there are similar (slightly smaller) local maxima.