Pearson Geometry Common Core, 2011
PG
Pearson Geometry Common Core, 2011 View details
6. Surface Areas and Volumes of Spheres
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Exercise 45 Page 738

Practice makes perfect
a Recall the standard equation of a circle with radius r and center ( h, k).
(x- h)^2+(y- k)^2= r^2 The standard equation of a sphere with radius r and center ( h, k, l) is very similar to the standard equation of a circle.

(x- h)^2+(y- k)^2+(z- l)^2= r^2 In our case, the sphere has center ( 0, 0, 0), and a radius of 5. (x- h)^2+(y- k)^2+(z- l)^2= r^2 ⇓ (x- 0)^2+(y- 0)^2+(z- 0)^2= 5^2 ⇕ x^2+y^2+z^2=25 Now, let us write the conditions for checking the relative position of a point and the sphere.

  • A point ( a, b, c) lies on the sphere x^2+ y^2+ z^2=25 when a^2+ b^2+ c^2=25.
  • A point ( a, b, c) lies inside the sphere x^2+ y^2+ z^2=25 when a^2+ b^2+ c^2<25.
  • A point ( a, b, c) lies outside the sphere x^2+ y^2+ z^2=25 when a^2+ b^2+ c^2>25.
We have to check the position of point A( 0, - 3, 4) with respect to the sphere. Let's substitute the values into the equation of the sphere.
x^2+ y^2+ z^2 ? 25
0^2+( - 3)^2+ 4^2 ? 25
0+9+16 ? 25
25 ? 25
25 =25
Therefore, point A lies on the sphere.
b From Part A we know that the given sphere has the equation x^2+y^2+z^2=25. Now, let's recall the conditions for checking the relative position of a point and the sphere.
  • A point ( a, b, c) lies on the sphere x^2+ y^2+ z^2=25 when a^2+ b^2+ c^2=25.
  • A point ( a, b, c) lies inside the sphere x^2+ y^2+ z^2=25 when a^2+ b^2+ c^2<25.
  • A point ( a, b, c) lies outside the sphere x^2+ y^2+ z^2=25 when a^2+ b^2+ c^2>25.
We have to check the position of point B( 1, - 1, - 1) with respect to the sphere. Let's substitute the values into the equation of the sphere.
x^2+ y^2+ z^2 ? 25
1^2+( - 1)^2+( - 1)^2 ? 25
1+1+1 ? 25
3 ? 25
3 <25
Therefore, point B lies inside the sphere.
c From Part A we know that the given sphere has the equation x^2+y^2+z^2=25. Now, let's recall the conditions for checking the relative position of a point and the sphere.
  • A point ( a, b, c) lies on the sphere x^2+ y^2+ z^2=25 when a^2+ b^2+ c^2=25.
  • A point ( a, b, c) lies inside the sphere x^2+ y^2+ z^2=25 when a^2+ b^2+ c^2<25.
  • A point ( a, b, c) lies outside the sphere x^2+ y^2+ z^2=25 when a^2+ b^2+ c^2>25.
We have to check the position of point C( 4, - 6, - 10) with respect to the sphere. Let's substitute the values into the equation of the sphere.
x^2+ y^2+ z^2 ? 25
4^2+( - 6)^2+( - 10)^2 ? 25
16+36+100 ? 25
152 ? 25
152 >25
Therefore, point C lies outside the sphere.