Question #288908

1.  An archeologist found the remains of an ancient wheel, which she then placed on a grid. If an arc of the wheel passes through A(-7,0), B(-3,4) and C(7,0), located the center of the wheel, and the standard equation of the circle defining its boundary. 


1
Expert's answer
2022-01-20T10:53:16-0500

The standard equation of the circle defining its boundary is



(xa)2+(yb)2=R2(x-a)^2 + (y-b)^2 = R^2

where the point with the coordinates (a, b) is the center of the circle. Put each point in the equation. Then we get the system of equations:



{(7a)2+(0b)2=R2,(3a)2+(4b)2=R2,(7a)2+(0b)2=R2.\left\{ \begin{array}{rcl} {(-7-a)^2 + (0 - b)^2 = R^2,} \\ {(-3-a)^2 + (4 - b)^2 = R^2,} \\ {(7-a)^2 + (0 - b)^2 = R^2.} \end{array} \right.

Open brackets (use the formula (a+b)2=a2+b2+2ab(a+b)^2 = a^2 + b^2 + 2ab) :



{49+a2+14a+b2=R2,9+a2+6a+16+b28b=R2,49+a214a+b2=R2.\left\{ \begin{array}{rcl} {49 +a^2+ 14a + b^2 = R^2,} \\ {9+a^2 +6a+ 16+ b^2 - 8b = R^2,} \\ {49 +a^2- 14a + b^2 = R^2.} \end{array} \right.

Subtract from the first equation the third:

49+a2+14a+b2(49+a214a+b2)=R2R249 +a^2+ 14a + b^2 - (49 +a^2- 14a + b^2) = R^2 -R^2

28a=0    a=028a = 0 \implies a = 0

Equate the first equation to the second:

49+a2+14a+b2=9+a2+6a+16+b28b49 +a^2+ 14a + b^2 = 9+a^2 +6a+ 16+ b^2 - 8b

Because a=0a = 0:

49+b2=9+16+b28b49 +b^2 = 9+16+ b^2 - 8b

258b=498b=24    b=325 - 8b = 49 -8b = 24 \implies b = -3

Then the center of the circle is (0, -3).

Find the radius of the circle: put point and the coordinates of center in the equation:

(70)2+(0(3))2=R2(-7-0)^2 + (0 - (-3))^2 = R^2

49+9=R2    R=5849 +9 = R^2 \implies R =\sqrt{58}

The standard equation of the circle:



x2+(y+3)2=58x^2 + (y+3)^2 = 58

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