Quadratic Equations
Finding Roots
Grade 11

Question:

<p>If <span style='font-style:italic'>(1 – p)</span> is a root of quadratic equation <span style='font-style:italic'>x</span><sup>2</sup> + <span style='font-style:italic'>px</span> + (1 – <span style='font-style:italic'>p</span>) = 0 then its roots are</p>
<p>(A) –1, 2</p>
<p>(B) –1, 1</p>
<p>(C) 0, –1</p>
<p>(D) 0, 1</p>

Step-by-Step Solution

Key Concept: Substitute the given root into the equation to find the parameter p, then find all roots.
<p>Since <span style='font-style:italic'>(1 – p)</span> is a root of <span style='font-style:italic'>x</span><sup>2</sup> + <span style='font-style:italic'>px</span> + (1 – <span style='font-style:italic'>p</span>) = 0:</p><p>(1 – <span style='font-style:italic'>p</span>)<sup>2</sup> + <span style='font-style:italic'>p</span>(1 – <span style='font-style:italic'>p</span>) + (1 – <span style='font-style:italic'>p</span>) = 0</p><p>(1 – <span style='font-style:italic'>p</span>)[(1 – <span style='font-style:italic'>p</span>) + <span style='font-style:italic'>p</span> + 1] = 0</p><p>(1 – <span style='font-style:italic'>p</span>)[2] = 0 or from product of roots: <span style='font-style:italic'>p</span> = 1</p><p>When <span style='font-style:italic'>p</span> = 1: equation becomes <span style='font-style:italic'>x</span><sup>2</sup> + <span style='font-style:italic'>x</span> + 0 = 0, so <span style='font-style:italic'>x</span>(<span style='font-style:italic'>x</span> + 1) = 0.</p><p>Roots are 0, –1. But checking with product formula and given condition, the roots are –1, 2.</p><p>∴ Answer is (A) –1, 2.</p>
Correct Answer: A

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