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Find a unit vector in the direction in which f decreases most rapidly at (1,1,-1)?

Let f(x,y,z) = ye^(y-x)-z^2.

The gradient of f(x,y,z) = -ye^(y-x)i+ye^(y-x)j-2zk

The tangent plane of this at (1,1,-1) would be -1(x-1)i+1(y-1)j+2(z+1)k.

Question is, how can I find the unit vector with these values?

2 Answers

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  • 8 years ago
    Favorite Answer

    f(x,y,z) = ye^(y - x) - z²

    Remember when finding the partial derivative with respect to y to use the product rule:

    ∇f = <-ye^(y - x), ye^(y - x) + e^(y - x), 2z>

    = <-ye^(y - x), e^(y - x)(y + 1), 2z>

    At the point (1, 1, -1), we have:

    ∇f(1, 1, -1) = <-1, 2, -2>

    And the magnitude is given by:

    |∇f(1, 1, -1)| = √(1 + 4 + 4) = 3

    The unit vector in which f is decreasing most rapidly is given by:

    u = ∇f(1, 1, -1) / |∇f(1, 1, -1)|

    = <-1, 2, -2> / 3

    = <-1/3, 2/3, -2/3>

  • ?
    Lv 4
    4 years ago

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