How to draw decision boundary once we get the value of theta?

Question

I am solving the a exercise given by stanford university , machine learning course by Andrew Ng. Question is ( http://openclassroom.stanford.edu/MainFolder/DocumentPage.php?course=MachineLearning&doc=exercises/ex4/ex4.html ) to plot a decision boundary on the given data and then estimate the result of the new data . I have used the fminunc function to calculate the theta .

I have got three theta value's . But for drawing a line we will need only two theta , then what is the need of the third theta value.

I am unable to plot the decision boundary . please help me out .

Answer 1

It is said you added an intercept term thus it is normal for theta to have three dimension: two gradients (according X and Y plus the bias term). If theta_1 is the bias term, theta_2 the X gradient and theta_3 the Y gradient, the equation of your line is:

Y = -theta_2 / theta_3 .* X + theta_1

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