first commit with current lessons

10-04/plotQ.jl

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+using LinearAlgebra
+using Plots
+
+function plotQ(Q::Matrix, q::Vector, xyrange::Tuple{Tuple{Number, Number}, Tuple{Number, Number}})::Plots.Plot
+    f(x, y) = dot((1/2) * hcat(x, y) * Q, vcat(x, y)) + dot(q', vcat(x, y))
+    xrange = LinRange(xyrange[1][1], xyrange[2][1], 200)
+    yrange = LinRange(xyrange[1][2], xyrange[2][2], 200)
+    z = @. f(xrange', yrange)
+    plt = contour(xrange, yrange, z)
+    plt
+end