tolong bantu bab lingkaran

[tex]x {}^{2} + {y}^{2} + 4x - 4y - 9 = 0 \: \\ a = 4 \:b = - 4 \: c = - 9 \\ jari \: jari \: (r) = \sqrt{ \frac{1}{4}( {a})^{2} + \frac{1}{4} ({b})^{2} - c } \\ r = \sqrt{ \frac{1}{4}(4) { }^{2} + \frac{1}{4} ( { - 4})^{2} - ( - 9) } \\ r = \sqrt{ \frac{1}{4}(16) + \frac{1}{4}(16) + 9 } \\ r = \sqrt{4 + 4 + 9} \\ r = \sqrt{17} \\ luas \: lingkaran \: = \pi \times r {}^{2} \\ luas = 3.14 \times { \sqrt{17} }^{2} \\ luas = 3.14 \times 17 \\ luas = 53.38[/tex]