Write an equation for the graph shown below. Use y as the dependent variable.?

y = -9 sin(2x)

From the curve y = sinx, we can reflect about the x-axis to get:

y = -sinx

Then, stretching in the y direction by 9 (increasing amplitude) we get:

y = -9sinx

If this was correct, then we would have minimum at (π/2, -9) and maximum at (3π/2, 9). Also, intercepts at (0, 0), (π, 0) and (2π, 0)

However, we have minimum at (π/4, -9) and maximum at (3π/4, 9). Also, intercepts at (0, 0), (π/2, 0) and (π, 0)  

so, y = -9sinx has been squashed along the x axis by 1/2

Therefore, y = -9sin(2x) is our graph

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