Circles
Distance from external point to circle
Grade 11
Question:
<p>Given the equation of the circle is <span class="math inline">\(x^2 + y^2 + 4x - 10y - 7 = 0\)</span>, find the minimum distance from the point <span class="math inline">\((4, -3)\)</span> to any point on the circle.</p>
Step-by-Step Solution
Key Concept: The minimum distance from an external point to a circle equals the distance from the point to the centre minus the radius.
<p><strong>Step 1:</strong> Convert the equation to standard form to find centre and radius.</p><p><strong>Step 2:</strong> Centre is <span class="math inline">$(-2, 5)$</span> and radius <span class="math inline">$r = \sqrt{4 + 25 + 7} = \sqrt{36} = 6$</span></p><p><strong>Step 3:</strong> Find distance from point <span class="math inline">$(4, -3)$</span> to centre <span class="math inline">$(-2, 5)$</span>.</p><p><strong>Step 4:</strong> <span class="math inline">$d = \sqrt{(-2 - 4)^2 + (5 + 3)^2} = \sqrt{36 + 64} = \sqrt{100} = 10$</span></p><p><strong>Step 5:</strong> Verify the point lies outside the circle: <span class="math inline">$(4)^2 + (-3)^2 + 4(4) - 10(-3) - 7 = 64 > 0$</span></p><p><strong>Step 6:</strong> Minimum distance = distance to centre - radius = <span class="math inline">$10 - 6 = 4$</span></p><p>∴ Answer is <span class="math inline">$4$</span>.</p>
Correct Answer: 4