The function 𝑓(𝑥,𝑦)=𝑥3−3𝑥−2𝑥𝑦−𝑦2−2𝑦+4 has two critical points. Find and classify them. the left-most stationary point (the one with the lower value of x) is located at x = and y = and is a point. • the right-most stationary point (the one with the higher value of x) is located at x = and y = and is a point
Question
The function
𝑓(𝑥,𝑦)=𝑥3−3𝑥−2𝑥𝑦−𝑦2−2𝑦+4
has two critical points. Find and classify them.
the left-most stationary point (the one with the lower value of x) is located
at x = and y = and is a point.
• the right-most stationary point (the one with the higher value of x) is located
at x = and y = and is a point
Solution
To find the critical points of the function, we first need to find the partial derivatives of the function with respect to x and y, and then set them equal to zero.
The partial derivative of f with respect to x is:
f_x = 3x^2 - 3 - 2y
Setting this equal to zero gives us:
3x^2 - 3 - 2y = 0
The partial derivative of f with respect to y is:
f_y = -2x - 2y - 2
Setting this equal to zero gives us:
-2x - 2y - 2 = 0
Solving these two equations simultaneously will give us the critical points of the function.
From the second equation, we can express y in terms of x:
y = -x - 1
Substituting this into the first equation gives us:
3x^2 - 3 - 2(-x - 1) = 0 3x^2 - 3 + 2x + 2 = 0 3x^2 + 2x - 1 = 0
Solving this quadratic equation gives us:
x = [-2 ± sqrt((2)^2 - 43(-1))]/(2*3) x = [-2 ± sqrt(4 + 12)]/6 x = [-2 ± sqrt(16)]/6 x = [-2 ± 4]/6 x = 1/3, -1
Substituting x = 1/3 into y = -x - 1 gives y = -1/3 - 1 = -4/3 Substituting x = -1 into y = -x - 1 gives y = 1 - 1 = 0
So the critical points are (1/3, -4/3) and (-1, 0).
To classify these points, we need to find the second partial derivatives of the function and evaluate them at the critical points.
The second partial derivative of f with respect to x is:
f_xx = 6x
The second partial derivative of f with respect to y is:
f_yy = -2
The mixed partial derivative of f with respect to x and y is:
f_xy = -2
The determinant of the Hessian matrix at a point (x, y) is given by:
D = f_xx * f_yy - (f_xy)^2
Evaluating D at the point (1/3, -4/3) gives:
D = 6*(1/3) * -2 - (-2)^2 D = -4 - 4 D = -8
Since D < 0, the point (1/3, -4/3) is a saddle point.
Evaluating D at the point (-1, 0) gives:
D = 6*(-1) * -2 - (-2)^2 D = 12 - 4 D = 8
Since D > 0 and f_xx < 0, the point (-1, 0) is a local maximum.
So, the left-most stationary point (the one with the lower value of x) is located at x = -1 and y = 0 and is a local maximum. The right-most stationary point (the one with the higher value of x) is located at x = 1/3 and y = -4/3 and is a saddle point.
Similar Questions
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