Use polar coordinates to find the volume of the given solid. bounded by the paraboloid z = 9 + 2x2 + 2y2 and the plane z = 15 in the first octant

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Answer:

Z = 8 + 2x2 + 2y2

Convert to polar coordinates

Z = 8 + 2r2

Now theta will go from 0 to pi/2 because it's in the first quadrant.

R will go from 0 to the radius of the circle formed at the intersection of the plane and the paraboloid.

14 = 8 + 2r2

r = sqrt(3)

So r goes from 0 to sqrt(3).

You integrate 14-z where 0<r<sqrt(3) and 0<theta<pi/2.

It is 14-z and not z because just z would give the volume under the paraboloid.

Step-by-step explanation: please go answer my recent question

To answer this question it is necessary:

  • Determine the integration limits
  • Solve the resulting double integral after changing to polar coordinates.

The solution is:

V =  (9/4)× π cubic units

z = 9 + 2×x² + 2×y²             and the plane  z = 15

To find integration limits we look the interseption poitns so

15 = 9  + 2×x² + 2×y²  

6  = 2×x² + 2×y²             ⇒   3 = x² + y²  = r²     ⇒ r = √3

Then  r     ⇒    0 ≤  r  ≤ √3  

The question is about the volume in the first octant, then  

0  ≤  θ  ≤ π/2

Now :

x = r×cos θ       y  = r×sin θ    and dA = rdrdθ

V = ∫∫ (  3 -  r²×cos²θ + r²×sin²θ ) ×rdrdθ

V = ∫∫ ( 3 -  r² )×r dr dθ

V = ∫ (3 - r² )× rdr  ∫ dθ

V = (3/2)×r² - r⁴/4 | ( 0  √3 )   = (9/2) - (9/4)

V =   (9/2) × π/2

V =  (9/4)× π

V =  (9/4)× π cubic units

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