Pearson Geometry Common Core, 2011
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Pearson Geometry Common Core, 2011 View details
5. Circles in the Coordinate Plane
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Exercise 24 Page 801

Use the Distance Formula to calculate the radius of the circle.

(x+1)^2+(y+4)^2=25

Practice makes perfect
To begin, let's recall the Distance Formula. This formula is used to find the distance d between two points (x_1,y_1) and (x_2,y_2). d = sqrt((x_2-x_1)^2 + (y_2-y_1)^2) We will find the radius of our circle by substituting the given points into this formula and finding the distance between the center and the known point through which the circle passes.
d = sqrt((x_2-x_1)^2 + (y_2-y_1)^2)
d=sqrt(( -4-( -1))^2+( 0-( -4))^2)
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Evaluate right-hand side
d=sqrt((-4+1)^2+(0+4)^2)
d=sqrt((-3)^2+4^2)
d=sqrt(9+16)
d=sqrt(25)
d=5
Let's now recall the standard form of an equation of a circle. (x- h)^2+(y- k)^2= r^2 In this formula, ( h, k) is the center of the circle and r is its radius. We are told that the center of the circle is ( -1, -4). This information, together with r= 5, is enough to write the equation. (x-( -1))^2+(y-( -4))^2= 5^2 ⇕ (x+1)^2+(y+4)^2=25