6.2_ex5

当只有一个∑ 时，注意考虑到是矩阵，所以不用像神经网络一样sum

hx = X*theta;
J = (1/(2*m))*(sum((hx-y).^2));
reg = lambda/(2*m)*sum(theta(2:end).^2);
J = J + reg;

grad = (X‘ * (hx-y)) / m;
grad(2:end) = grad(2:end)+ lambda * theta(2:end) / m;