Core Connections Algebra 2, 2013
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Core Connections Algebra 2, 2013 View details
1. Section 8.1
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Exercise 59 Page 388

Practice makes perfect
a Let's start by recalling the standard equation of a circle.
(x- h)^2+(y- k)^2= r^2Here, the center is the point ( h, k) and the radius is r. We will rewrite the given equation to match this form and then identify the center and the radius. (y-7)^2=25-(x-3)^2 In this case, we are already given two squared binomials. In order to obtain the standard equation we should add (x-3)^2 to both sides of the equation.
(y-7)^2=25-(x-3)^2
(x-3)^2+(y-7)^2=25
(x- 3)^2+(y- 7)^2= 5^2
We obtained the standard equation of the circle, so we can identify its center and radius. The center of the circle is ( 3, 7) and its radius is 5.
b Examining the given circle, we can see that there are two terms containing y-variable.
x^2+ y^2+10 y=-9 To write the standard equation of the circle, we will need to complete the square for y. In this case, we should add 5^2 to both sides of the equation.
x^2+y^2+10y=-9
â–Ľ
Rewrite
x^2+y^2+10y+5^2=-9+5^2
x^2+y^2+2(y)(5)+5^2=-9+5^2
x^2+(y+5)^2=-9+5^2
x^2+(y+5)^2=-9+25
x^2+(y+5)^2=16
(x-0)^2+(y+5)^2=16

a+b=a-(- b)

(x-0)^2+(y-(-5))^2=16
(x- 0)^2+(y-( -5))^2= 4^2
Now we can see that the center of the given circle is ( 0, -5) and its radius is 4.
c Once again, we want to write the standard equation of the given circle.
x^2+y^2+18x-8y+47=0 This time, we will need to complete the square twice — once for each variable.
x^2+y^2+18x-8y+47=0
â–Ľ
Rewrite
x^2+y^2+18x-8y+81+16-50=0
x^2+18x+81+y^2-8y+16-50=0
x^2+18x+9^2+y^2-8y+4^2-50=0
x^2+2(x)(9)+9^2+y^2-2(y)(4)+4^2-50=0
(x+9)^2+y^2-2(y)(4)+4^2-50=0
(x+9)^2+(y-4)^2-50=0
(x+9)^2+(y-4)^2=50
(x+9)^2+(y-4)^2=(sqrt(50))^2
(x+9)^2+(y-4)^2=(sqrt(25*2))^2
(x+9)^2+(y-4)^2=(sqrt(25)*sqrt(2))^2
(x+9)^2+(y-4)^2=(5sqrt(2))^2

a+b=a-(- b)

(x-( -9))^2+(y- 4)^2=( 5sqrt(2))^2
From the standard equation of the given circle, we can see that its center is ( -9, 4) and its radius is 5sqrt(2).
d Similarly as in Part A, the squares are already given. However, let's rearrange the sum on the left-hand side by using Commutative Property of Addition to obtain the standard equation of the circle.

y^2+(x-3)^2=1 ⇕ (x- 3)^2+(y- 0)^2= 1^2 The center of the given circle is ( 3, 0) and its radius is 1.