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NeedHelp27

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When I tried to go about doing this, I drew a graph showing the person's height, at it ended up being the cosine graph shifted up 5 and over so that a low point was on the y-axis (0,5). Next, I tried to write the equation in the form f(x)=acos(bx+c)+d. I did (max value - min value)/2 = (55-5)/2=25 to find a. Then, because one revolution takes 40 seconds, I solved 2pi/b=40 for b and got b=pi/20. The graph is shifted 5 up, so d-5. That gave me f(x)=25cos((pix)/20+c)+5. I also have the point (0,5) on the graph, so I can plug it into get c. f(0)=5=25cos(c)+5. C ended up being pi/2.

Then I went to solve the problem.

f(t)=50=25cos((pix)/20+pi/2)+5.

When I finally isolated x, I got a domain error. Where did I go wrong?