Question

How do I use the following cosine equation to get the Sinusoid Max & Min Times (x values), and Sinusoid Max and Min Values (y values) in order graph a Tidal Wave Chart?

y = 11.412 cos ((5π / 31)(x-3:12)) +174.91

Answer 1

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Answer:

• maximum: (x, y) = (12.4n+3.2, 186.322)
• minimum: (x, y) = (12.4n+9.4, 163.498)

Explanation:

You know that cos(α) is a maximum at α=0, 2π, 4π, and all even multiples of π. You know cos(α) is a minimum for α=π, 3π, 5π, and all odd multiples of π.

You can find your value of x at which y will be a maximum by setting the argument of the cosine function equal to zero (and/or 2nπ). If we use α=2nπ, then we have ...

α = (5π/31)(x -3.2) = 2nπ

(x -3.2) = (31/5)(2n) = 12.4n

Tidal maxima will occur at ...

x = 12.4n +3.2 . . . . . for integer values of n

Without bothering to go through the solution for α being odd multiples of π, we can see from this that the period is 12.4 hours. We know the tidal minimum will be half a period later, or 6.2 hours later than this.

Tidal minima will occur at ...

x = 12.4n +9.4 . . . . for integers n

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Of course, cos(α) has extremes of ±1, so your tidal maximum will be ...

y = 11.412 +174.91 = 186.322

and your tidal minimum will be ...

y = -11.412 +174.91 = 163.498