The Equation of a CircleCentre, radius, and the completing-the-square step that…10th GradeA circle with centre (h, k) and radius r hasequation (x − h)² + (y − k)² = r².(x − h)² + (y − k)² = r²(x−2)² + (y+3)² = 25centre (2,−3), r = 5Note the sign flip on k.x² + y² = 9centre (0,0), r = 3Centred at the origin.r² = 25r = 5The equation stores r², not r.Expanded formcomplete the square twiceOnce for the x terms, once for the y terms.THE COMMON MISTAKE✗ reading r as 25✓ r = 5The right-hand side is r squared. Take the root.It is the distance formula rearranged — every point exactly r fromthe centre.The Equation of a Circlewww.littleactivity.com

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Math · 10th Grade

The Equation of a Circle

Centre, radius, and the completing-the-square step that reveals both.

How to use it

If it is not in standard form, complete the square in x and in y separately.

Free · print-ready PDF · US Letter and A4

Grade
10th Grade
Ages
15–16
Subject
Math
Pages
1

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