NIMCET 2007 — Mathematics PYQ
NIMCET | Mathematics | 2007Find the term in the expansion of :

Find the 6th term in the expansion of (1−2x2)1/2:
87x5
−87x5
−87x10
(Correct Answer)32−693x10
−87x10
According to the binomial theorem for any rational index n, the expansion of (1−y)n is given by:
(1−y)n=1−ny+2!n(n−1)y2−3!n(n−1)(n−2)y3+…
The (r+1)th general term in the binomial expansion of (1−y)n is:
Tr+1=r!n(n−1)(n−2)…(n−r+1)(−y)r
Alternatively, comparing directly with (1+X)n where X=−2x2:
Tr+1=r!n(n−1)(n−2)…(n−r+1)Xr
From the given expression (1−2x2)1/2:
X=−2x2
Index (n) = 21
We need to find the 6th term (T6). This means:
r+1=6⟹r=5
Substitute n=21, r=5, and X=−2x2 into the general term equation:
T6=5!21(21−1)(21−2)(21−3)(21−4)(−2x2)5
Let's simplify the bracket components individually:
(21−1)=−21
(21−2)=−23
(21−3)=−25
(21−4)=−27
Now combine them inside the numerator expression:
Numerator=(21)(−21)(−23)(−25)(−27)=251×(−1)×(−3)×(−5)×(−7)
Since there are 4 negative signs, the product turns positive:
Numerator=32105
Now compute the factorial and power elements:
5! = 5 \times 4 \times 3 \times 2 \times 1 = 120<span class="katex-error" title="ParseError: KaTeX parse error: Expected 'EOF', got '&' at position 43: …="font-family: &̲quot;Google San…" style="color:#cc0000"></p></li><li><p><span style="font-family: &quot;Google Sans Text&quot;, sans-serif !important;">(-2x^2)^5 = (-2)^5 \cdot (x^2)^5 = -32x^{10}</span></p></li></ul><h3><strong>Step 5: Put It All Together</strong></h3><p>Substitute the evaluated parts back into the expression for <span style="font-family: &quot;Google Sans Text&quot;, sans-serif !important;">T_6</span>:</p><p></span>T_6 = \frac{\frac{105}{32}}{120} \times (-32x^{10})<span class="katex-error" title="ParseError: KaTeX parse error: Expected 'EOF', got '&' at position 56: …="font-family: &̲quot;Google San…" style="color:#cc0000"></p><p>Cancel out the common <span style="font-family: &quot;Google Sans Text&quot;, sans-serif !important;">32</span> from the numerator and denominator:</p><p></span>T_6 = \frac{105}{120} \times (-1) \times x^{10}<span class="katex-display"><span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.5782em;vertical-align:-0.0391em;"></span><span class="mrel"><</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">/</span><span class="mord mathnormal">p</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">><</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:0.7335em;vertical-align:-0.1944em;"></span><span class="mord mathnormal">p</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">></span></span></span></span></span>T_6 = -\frac{105}{120} x^{10}<span class="katex-error" title="ParseError: KaTeX parse error: Expected 'EOF', got '&' at position 139: …="font-family: &̲quot;Google San…" style="color:#cc0000"></p><p>Reduce the fraction by dividing both the numerator and the denominator by their greatest common divisor (<span style="font-family: &quot;Google Sans Text&quot;, sans-serif !important;">15</span>):</p><p></span>T_6 = -\frac{105 \div 15}{120 \div 15} x^{10} = -\frac{7}{8} x^{10}$
According to the binomial theorem for any rational index n, the expansion of (1−y)n is given by:
(1−y)n=1−ny+2!n(n−1)y2−3!n(n−1)(n−2)y3+…
The (r+1)th general term in the binomial expansion of (1−y)n is:
Tr+1=r!n(n−1)(n−2)…(n−r+1)(−y)r
Alternatively, comparing directly with (1+X)n where X=−2x2:
Tr+1=r!n(n−1)(n−2)…(n−r+1)Xr
From the given expression (1−2x2)1/2:
X=−2x2
Index (n) = 21
We need to find the 6th term (T6). This means:
r+1=6⟹r=5
Substitute n=21, r=5, and X=−2x2 into the general term equation:
T6=5!21(21−1)(21−2)(21−3)(21−4)(−2x2)5
Let's simplify the bracket components individually:
(21−1)=−21
(21−2)=−23
(21−3)=−25
(21−4)=−27
Now combine them inside the numerator expression:
Numerator=(21)(−21)(−23)(−25)(−27)=251×(−1)×(−3)×(−5)×(−7)
Since there are 4 negative signs, the product turns positive:
Numerator=32105
Now compute the factorial and power elements:
5! = 5 \times 4 \times 3 \times 2 \times 1 = 120<span class="katex-error" title="ParseError: KaTeX parse error: Expected 'EOF', got '&' at position 43: …="font-family: &̲quot;Google San…" style="color:#cc0000"></p></li><li><p><span style="font-family: &quot;Google Sans Text&quot;, sans-serif !important;">(-2x^2)^5 = (-2)^5 \cdot (x^2)^5 = -32x^{10}</span></p></li></ul><h3><strong>Step 5: Put It All Together</strong></h3><p>Substitute the evaluated parts back into the expression for <span style="font-family: &quot;Google Sans Text&quot;, sans-serif !important;">T_6</span>:</p><p></span>T_6 = \frac{\frac{105}{32}}{120} \times (-32x^{10})<span class="katex-error" title="ParseError: KaTeX parse error: Expected 'EOF', got '&' at position 56: …="font-family: &̲quot;Google San…" style="color:#cc0000"></p><p>Cancel out the common <span style="font-family: &quot;Google Sans Text&quot;, sans-serif !important;">32</span> from the numerator and denominator:</p><p></span>T_6 = \frac{105}{120} \times (-1) \times x^{10}<span class="katex-display"><span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.5782em;vertical-align:-0.0391em;"></span><span class="mrel"><</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">/</span><span class="mord mathnormal">p</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">><</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:0.7335em;vertical-align:-0.1944em;"></span><span class="mord mathnormal">p</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">></span></span></span></span></span>T_6 = -\frac{105}{120} x^{10}<span class="katex-error" title="ParseError: KaTeX parse error: Expected 'EOF', got '&' at position 139: …="font-family: &̲quot;Google San…" style="color:#cc0000"></p><p>Reduce the fraction by dividing both the numerator and the denominator by their greatest common divisor (<span style="font-family: &quot;Google Sans Text&quot;, sans-serif !important;">15</span>):</p><p></span>T_6 = -\frac{105 \div 15}{120 \div 15} x^{10} = -\frac{7}{8} x^{10}$
