nonlinear-functions

I have several data points that are plotted below and I would like to find the frequency value when the amplitude value crosses 4 . I've included an example along with the data points in the example below. I've circled the answer graphically but I'm not sure how to compute it mathematically and get all the values for the frequencies I desire. How can I do this with octave / matlab? Also is there a mathematical term for what I'm trying to do? In this example I'm trying to get 5 frequencies (but this is just an example) I know two answers are 30 and 80 but not sure how to get the rest. The full

I have 4 non-linear equations with three unknowns X , Y , and Z that I want to solve for. The equations are of the form: F(m) = X^2 + a(m)Y^2 + b(m)XYcosZ + c(m)XYsinZ ...where a , b and c are constants which are dependent on each value of F in the four equations. What is the best way to go about solving this? There are two ways to do this. Use a non-linear solver Linearize the problem and solve it in the least-squares sense Setup So, as I understand your question, you know F, a, b, and c at 4 different points, and you want to invert for the model parameters X, Y, and Z. We have 3 unknowns and