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The equation x^2+y^2-4x+2y=b describes a circle. Determine the y-coordinate of the center of the circle.

The radius of the circle is 7 units. What is the value of b in the equation?

Verified Answer :

x² + y² - 4 x + 2 y = b
x² - 4 x + 4 + y² + 2 y + 1 = b + 4 + 1
( x - 2 )² + ( y + 1 )² = b + 5
y - coordinate of the center of the circle is:  y = - 1
b + 5 = r
b + 5 = 7
b = 7 - 5
b = 2

Answer:

Step-by-step explanation:

The equation is given as:

\(x^2+y^2-4x+2y=b\)

Upon solving this equation, we have

\(x^2-4x+4+y^2+2y+1=b+4+1\)

which can be written as:

\((x-2)^2+(y+1)^2=b+5\)                   (1)

Thus, the y- coordinate of the center of the circle is \(y=-1\).

Now, comparing the equation (1) with the equation of circle, we have

\((x-a)^2+(y-c)^2=r^2\)

where r is the radius of the circle and (a,c) is the center.

Thus, on comparing, we have

⇒\(b+5=r^2\)

Also, it is given that the radius of the circle is 7 units, thus putting r=7 in above equation, we get

⇒\(b+5=(7)^2\)

⇒\(b+5=49\)

⇒\(b=44 units\)

Thus, the value of b is 44 units in the given equation.