Area Enclosed by a Circle Equation

Complete the square: (x − 5)² + (y + 3)² = −18 + 25 + 9 = 16, so r = 4. The area is πr² = 16π ≈ 50.3. A common slip is to use r² = 25 + 9 + 18 = 52 by forgetting that +18 moves to the right side as −18. TI-36X Pro tip: store the center (5 to x; (−)3 to y, using the (−) key, not minus) with sto→ and the x yzt abcd key, then evaluate x² + y² − 10x + 6y + 18 to get −16, so r² = 16. Then type 16 × π and press enter for 50.27.