Section 3.8

Differentiation of Implicit Functions

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Problem 1301

x3+y3−3axy=0x^{3}+y^{3}-3axy = 0. Find y′y' at x=yx=y.

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Remark.
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An equation f(x,y)=0f(x,y) = 0, having as a solution a pair of numbers (x0,y0)(x_{0},y_{0}), defines in a neighborhood of the number x0x_{0} the quantity yy as a function of xx, if the partial derivative ∂f∂y\frac{\partial f}{\partial y} is nonzero at the point (x0,y0)(x_{0},y_{0}) and continuous in a neighborhood of it. The successive derivatives of yy with respect to xx can be obtained from the equalities:

∂f∂x+∂f∂yy′=0, \frac{\partial f}{\partial x}+\frac{\partial f}{\partial y}y' = 0, ∂2f∂x2+2∂2f∂x∂yy′+∂2f∂y2y′2+∂f∂yy′′=0, \frac{\partial ^{2} f}{\partial x^{2}}+2\frac{\partial ^{2} f}{\partial x \partial y}y'+\frac{\partial ^{2} f}{\partial y^{2}}y'^{2}+\frac{\partial f}{\partial y}y'' = 0, ∂3f∂x3+3∂3f∂x2∂yy′+3∂3f∂x∂y2y′2+∂3f∂y3y′3+3∂2f∂x∂yy′′+3∂2f∂y2y′y′′+∂f∂yy′′′=0. \frac{\partial ^{3} f}{\partial x^{3}}+3\frac{\partial ^{3} f}{\partial x^{2} \partial y}y'+3\frac{\partial ^{3} f}{\partial x \partial y^{2}}y'^{2}+\frac{\partial ^{3} f}{\partial y^{3}}y'^{3}+3\frac{\partial ^{2} f}{\partial x \partial y}y''+3\frac{\partial ^{2} f}{\partial y^{2}}y'y''+\frac{\partial f}{\partial y}y''' = 0.

In a similar way one can also compute the derivatives of implicit functions given by a single equation in several variables.

If several implicit functions are given by several equations, then in computing the derivatives it is sometimes convenient to use differentials. Here one should keep in mind that the higher-order differentials of those variables taken as independent are equal to zero.

Problem 1302

x3+y3−3axy=0x^{3}+y^{3}-3axy = 0. Find xx for which y′=0y' = 0.

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Problem 1303

xy=yxx^{y} = y^{x}. Find y′y' for x≠yx \neq y.

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Problem 1304

xsin⁡y−cos⁡y+cos⁡2y=0x\sin y-\cos y+\cos 2y = 0. Find y′y'.

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Problem 1305

x=y−αsin⁡yx = y-\alpha \sin y. Find y′y' and y′′y''.

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Problem 1306

x2+2xy+y2−4x+2y−2=0x^{2}+2xy+y^{2}-4x+2y-2 = 0. Find y′′′y''' at x=1x=1, y=1y=1.

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Problem 1307

x+y=ex−yx+y = e^{x-y}. Find y′′y''.

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Problem 1308

ln⁡x2+y2=arctg⁡yx\ln \sqrt{x^{2}+y^{2}} = \operatorname {arctg} \frac{y}{x}. Find y′′y''.

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Problem 1309

(x−a)2+(y−b)2=R2(x-a)^{2}+(y-b)^{2} = R^{2}. Regarding xx here as the independent variable and yy as a function, take the fourth derivative of both sides of the equation.

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Problem 1310

For the equation x(x2+y2)−a(x2−y2)=0x(x^{2}+y^{2})-a(x^{2}-y^{2}) = 0 at x=0x=0, y=0y=0, the formula ∂f∂x+∂f∂yy′=0\frac{\partial f}{\partial x}+\frac{\partial f}{\partial y}y' = 0 does not make it possible to find y′y'; however, y′y' can be found here too, from the formula

∂2f∂x2+2∂2f∂x∂yy′+∂2f∂y2y′2+∂f∂yy′′=0. \frac{\partial ^{2} f}{\partial x^{2}}+2\frac{\partial ^{2} f}{\partial x \partial y}y'+\frac{\partial ^{2} f}{\partial y^{2}}y'^{2}+\frac{\partial f}{\partial y}y'' = 0.

Find y′y'.

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Problem 1311

(x2+y2−bx)2=a2(x2+y2)(x^{2}+y^{2}-bx)^{2} = a^{2}(x^{2}+y^{2}). Find y′y' at x=0x=0, y=0y=0.

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Problem 1312

The same question for the equation x3+y3−3xy=0x^{3}+y^{3}-3xy = 0.

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Problem 1313

The equations x2−y2+z2=1x^{2}-y^{2}+z^{2} = 1, y2−2x+z=0y^{2}-2x+z = 0 are given. Find y′y' and z′′z'' at x=1x=1, y=1y=1, z=1z=1.

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Problem 1314
{x3+y3+z3=a3,x2+y2+z2=b2. \begin{cases} x^{3}+y^{3}+z^{3} = a^{3}, \\ x^{2}+y^{2}+z^{2} = b^{2}. \end{cases}

Find y′y' and z′z'.

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Problem 1315

From the equations x2+y2−z2=0x^{2}+y^{2}-z^{2} = 0, x2+2y2+3z2=1x^{2}+2y^{2}+3z^{2} = 1, find d2yd^{2}y and d2zd^{2}z, if xx is the independent variable.

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Problem 1316

From the equations x2+y2=2z2x^{2}+y^{2} = 2z^{2}, x2+2y2+z2=4x^{2}+2y^{2}+z^{2} = 4, find dxdz\frac{dx}{dz} and d2ydz2\frac{d^{2}y}{dz^{2}} at the point (1,−1,1)(1,-1,1), if zz is the independent variable.

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Problem 1317

x+y+z=ax+y+z = a, x3+y3+z3=3xyzx^{3}+y^{3}+z^{3} = 3xyz. Find the derivatives of yy and zz.

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Problem 1318

At the point (1,1,−2)(1,1,-2), find the first and second derivatives of yy and zz, if x+y+z=0x+y+z = 0, x3+y3−z3=10x^{3}+y^{3}-z^{3} = 10.

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Problem 1319

x2+y2+z2=2zx^{2}+y^{2}+z^{2} = 2z. Find ∂2z∂x2\frac{\partial^{2} z}{\partial x^{2}}.

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Problem 1320

x3+y3+z3−3z=0x^{3}+y^{3}+z^{3}-3z = 0. Find ∂2z∂x∂y\frac{\partial^{2} z}{\partial x \partial y}.

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Problem 1321

x2−2y2+z2−4x+2z−5=0x^{2}-2y^{2}+z^{2}-4x+2z-5 = 0. Find ∂z∂x\frac{\partial z}{\partial x} and ∂2z∂x∂y\frac{\partial^{2} z}{\partial x \partial y}.

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Problem 1322

xcos⁡y+ycos⁡z+zcos⁡x=ax\cos y+y\cos z+z\cos x = a. Find ∂z∂x\frac{\partial z}{\partial x} and ∂z∂y\frac{\partial z}{\partial y}.

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Problem 1323

xy+xz+yz=1xy+xz+yz = 1. Find dzdz and d2zd^{2}z.

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Problem 1324

Find d2zd^{2}z, if x2a2+y2b2+z2c2=1\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}+\frac{z^{2}}{c^{2}} = 1.

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Problem 1325

Find d2zd^{2}z, if cos⁡2x+cos⁡2y+cos⁡2z=1\cos^{2} x+\cos^{2} y+\cos^{2} z = 1.

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Problem 1326

Find d2zd^{2}z at the point x=ax=a, y=ay=a, z=0z=0, if x3+z3−3axz=y3x^{3}+z^{3}-3axz = y^{3}.

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Problem 1327

Find the second partial derivatives of zz, if this function of xx and yy is defined by the equation y=xφ(z)+ψ(z)y = x\varphi (z)+\psi (z).

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Problem 1328

Show that x∂u∂x+y∂u∂y+z∂u∂z=0x\frac{\partial u}{\partial x}+y\frac{\partial u}{\partial y}+z\frac{\partial u}{\partial z} = 0, if uv=3x−2y+zuv = 3x-2y+z, v2=x2+y2+z2v^{2} = x^{2}+y^{2}+z^{2}.

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Problem 1329

xu+yv=0xu+yv = 0, uv−xy=5uv-xy = 5; at x=1x=1, y=−1y=-1 we take u=v=2u=v=2. Find ∂2u∂x2\frac{\partial^{2} u}{\partial x^{2}} and ∂2u∂x∂y\frac{\partial^{2} u}{\partial x \partial y}.

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Problem 1330

x=φ(t)x = \varphi (t), y=ψ(t)y = \psi (t), z=kt2z = kt^{2}. Find the derivatives of xx and yy with respect to zz.

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Problem 1331

x=acos⁡usin⁡vx = a\cos u \sin v, y=bcos⁡ucos⁡vy = b\cos u \cos v, z=csin⁡uz = c\sin u. Find ∂z∂x\frac{\partial z}{\partial x} and ∂z∂y\frac{\partial z}{\partial y}.

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Problem 1332

Show that if F(x,y,z)=0F(x,y,z) = 0, then ∂z∂x⋅∂x∂y⋅∂y∂z=−1\frac{\partial z}{\partial x} \cdot \frac{\partial x}{\partial y} \cdot \frac{\partial y}{\partial z} = -1.

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Problem 1333

The function uu is given by the formula u=x3y2zu = x^{3}y^{2}z, where the variables xx, yy, and zz are related by the equation x2+y2+z2=3xyzx^{2}+y^{2}+z^{2} = 3xyz. Determine ∂u∂x\frac{\partial u}{\partial x} at the point (1,1,1)(1,1,1), when:

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(a)

the independent variables are xx and yy;

(b)

the independent variables are xx and zz.

Problem 1334

Show that zz, defined as a function of xx and yy by the equation x−az=φ(y−bz)x-az = \varphi (y-bz), where φ\varphi is any differentiable function, satisfies the partial differential equation

a∂z∂x+b∂z∂y=1, a\frac{\partial z}{\partial x}+b\frac{\partial z}{\partial y} = 1,

called the equation of cylindrical surfaces.

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Problem 1335

Show that zz, given as a function of xx and yy by the equation z=xφ(zy)z = x\varphi \left(\frac{z}{y}\right), satisfies the equation of conical surfaces

x∂z∂x+y∂z∂y=z. x\frac{\partial z}{\partial x}+y\frac{\partial z}{\partial y} = z.
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Problem 1336

Show that, subject to the equations

z=αx+yφ(α)+ψ(α), z = \alpha x+y\varphi (\alpha )+\psi (\alpha ), 0=x+yφ′(α)+ψ′(α), 0 = x+y\varphi '(\alpha )+\psi '(\alpha ),

zz satisfies the equation

∂2z∂x2∂2z∂y2−(∂2z∂x∂y)2=0. \frac{\partial ^{2} z}{\partial x^{2}}\frac{\partial ^{2} z}{\partial y^{2}}-\left(\frac{\partial ^{2} z}{\partial x \partial y}\right)^{2} = 0.
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Problem 1337

Show that for y=xφ(z)+ψ(z)y = x\varphi (z)+\psi (z) the equation

∂2z∂x2(∂z∂y)2−2∂z∂x∂z∂y∂2z∂x∂y+∂2z∂y2(∂z∂x)2=0. \frac{\partial ^{2} z}{\partial x^{2}}\left(\frac{\partial z}{\partial y}\right)^{2}-2\frac{\partial z}{\partial x}\frac{\partial z}{\partial y}\frac{\partial ^{2} z}{\partial x \partial y}+\frac{\partial ^{2} z}{\partial y^{2}}\left(\frac{\partial z}{\partial x}\right)^{2} = 0.

is satisfied.

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Problem 1338

The function zz of xx and yy is given by the equations:

(z−φ(α))2=x2(y2−α2), (z-\varphi (\alpha ))^{2} = x^{2}(y^{2}-\alpha ^{2}), (z−φ(α))φ′(α)=αx2. (z-\varphi (\alpha ))\varphi '(\alpha ) = \alpha x^{2}.

Show that

∂z∂x∂z∂y=xy. \frac{\partial z}{\partial x}\frac{\partial z}{\partial y} = xy.
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Problem 1339

The equations are given:

z=ψ(α)(x+α)2ψ′(α)+1x+α,y+ln⁡((x+α)2ψ′(α))=0. z = \frac{\psi (\alpha )}{(x+\alpha )^{2}\psi '(\alpha )}+\frac{1}{x+\alpha }, \qquad y+\ln \left((x+\alpha )^{2}\psi '(\alpha )\right) = 0.

Show that

(z−∂z∂y)2+∂z∂x=0. \left(z-\frac{\partial z}{\partial y}\right)^{2}+\frac{\partial z}{\partial x} = 0.
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