Relations & Functions
Range and Domain
Grade 12
Question:
<p>Let <span class="math">g: [\frac{π}{2}, π] → A</span> defined by <span class="math">g(x) = \frac{\sin x + 4}{\sin x - 2}</span> be an invertible function. Find the set <span class="math">A</span>.</p>
<p>(a) <span class="math">[5, 2]</span></p>
<p>(b) <span class="math">[-2, 5]</span></p>
<p>(c) <span class="math">[-5, 2]</span></p>
<p>(d) <span class="math">[-5, -2]</span></p>
Step-by-Step Solution
Key Concept: Find the range of the function by analyzing how it behaves over the given domain. Evaluate at boundary points and determine monotonicity.
<p>Let <span class="math">y = \frac{\sin x + 4}{\sin x - 2}</span> where <span class="math">x ∈ [\frac{π}{2}, π]</span></p><p>On the interval <span class="math">[\frac{π}{2}, π]</span>, we have <span class="math">\sin x ∈ [0, 1]</span></p><p>For the numerator: <span class="math">\sin x + 4 ∈ [4, 5]</span></p><p>For the denominator: <span class="math">\sin x - 2 ∈ [-2, -1]</span></p><p>Since the numerator is positive and denominator is negative, <span class="math">y</span> is negative.</p><p>At <span class="math">x = \frac{π}{2}</span> (where <span class="math">\sin x = 1</span>): <span class="math">y = \frac{1 + 4}{1 - 2} = \frac{5}{-1} = -5</span></p><p>At <span class="math">x = π</span> (where <span class="math">\sin x = 0</span>): <span class="math">y = \frac{0 + 4}{0 - 2} = \frac{4}{-2} = -2</span></p><p>As <span class="math">\sin x</span> increases from 0 to 1, <span class="math">y</span> decreases from <span class="math">-2</span> to <span class="math">-5</span></p><p>Therefore: <span class="math">A = [-5, -2]</span></p><p>∴ Answer is (d).</p>
Correct Answer: D