Exam 14: Partial Derivatives

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Find the domain and range of the function g(x,y)=x2+2y2+3g ( x , y ) = x ^ { 2 } + 2 y ^ { 2 } + 3 .

(Multiple Choice)
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 Find three positive numbers whose sum is 400 and whose product is a maximum. \text { Find three positive numbers whose sum is } 400 \text { and whose product is a maximum. }

(Short Answer)
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Evaluate the limit. lim(x,y)(0,0)7xy2x2+y2\lim _ { ( x , y ) \rightarrow ( 0,0 ) } \frac { 7 x y ^ { 2 } } { x ^ { 2 } + y ^ { 2 } }

(Short Answer)
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 Find h(x,y)=g(f(x,y)), if g(t)=t2+t and f(x,y)=7x+6y6\text { Find } h ( x , y ) = g ( f ( x , y ) ) \text {, if } g ( t ) = t ^ { 2 } + \sqrt { t } \text { and } f ( x , y ) = 7 x + 6 y - 6

(Short Answer)
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Find and classify the relative extrema and saddle points of the function f(x,y)=e2xsin4yf ( x , y ) = e ^ { - 2 x } \sin 4 y for x0x \geq 0 and 0yπ2.0 \leq y \leq \frac { \pi } { 2 } .

(Short Answer)
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 Use the definition of partial derivatives as limits to find fx(x,y) if f(x,y)=5x29xy+2y2\text { Use the definition of partial derivatives as limits to find } f _ { x } ( x , y ) \text { if } f ( x , y ) = 5 x ^ { 2 } - 9 x y + 2 y ^ { 2 } \text {. }

(Short Answer)
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 Find the gradient of f(x,y,z)=x4eyz at the point P(2,0,9)\text { Find the gradient of } f ( x , y , z ) = x ^ { 4 } e ^ { y z } \text { at the point } P ( - 2,0 , - 9 )

(Short Answer)
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A contour map for a function ff is shown. Use it to estimate the value of f(3,3)f ( - 3,3 ) .  A contour map for a function  f  is shown. Use it to estimate the value of  f ( - 3,3 ) .

(Multiple Choice)
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 Find 2zx2 for z=ytan7x\text { Find } \frac { \partial ^ { 2 } z } { \partial x ^ { 2 } } \text { for } z = y \tan 7 x

(Short Answer)
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 Find the differential of the function z=3x3y6\text { Find the differential of the function } z = 3 x ^ { 3 } y ^ { 6 } \text {. }

(Short Answer)
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The ellipsoid 8x2+3y2+z2=148 x ^ { 2 } + 3 y ^ { 2 } + z ^ { 2 } = 14 intersects the plane y=2y = 2 in an ellipse. Find parametric equations for the tangent line to this ellipse at the point (1,2,2)( 1,2,2 ) .

(Multiple Choice)
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 Find three positive numbers whose sum is 400 and whose product is a maximum. \text { Find three positive numbers whose sum is } 400 \text { and whose product is a maximum. }

(Short Answer)
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If f(x,y)=x2+6y2f ( x , y ) = x ^ { 2 } + 6 y ^ { 2 } , use the gradient vector f(10,2)\nabla f ( 10,2 ) to find the tangent line to the level curve f(x,y)=124f ( x , y ) = 124 at the point (10,2)( 10,2 ) .

(Multiple Choice)
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Use differentials to estimate the amount of metal in a closed cylindrical can that is 12 cm12 \mathrm {~cm} high and 8 cm\mathrm { cm } in diameter if the metal in the top and bottom is 0.09 cm0.09 \mathrm {~cm} thick and the metal in the sides is 0.010.01 cm\mathrm { cm } thick. (rounded to the nearest hundredth.)

(Multiple Choice)
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Evaluate the limit. lim(x,y)(0,0)4xy2x2+y2\lim _ { ( x , y ) \rightarrow ( 0,0 ) } \frac { 4 x y ^ { 2 } } { x ^ { 2 } + y ^ { 2 } }

(Short Answer)
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Use the Chain Rule to find ws\frac { \partial w } { \partial s } where s=4,t=0s = 4 , t = 0 . w=x2+y2+z2,x=st,y=scost,z=ssintw = x ^ { 2 } + y ^ { 2 } + z ^ { 2 } , \quad x = s t , \quad y = s \cos t , \quad z = s \sin t

(Short Answer)
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Let g(r,s,t)=res/tg ( r , s , t ) = r e ^ { s / t } . Find g(9,ln5,12)g \left( 9 , \ln 5 , \frac { 1 } { 2 } \right) . Select the correct answer.

(Multiple Choice)
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Let g(r,s,t)=res/tg ( r , s , t ) = r e ^ { s / t } . Find g(4,ln3,12)g \left( 4 , \ln 3 , \frac { 1 } { 2 } \right) . Select the correct answer.

(Multiple Choice)
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