Question : If $\cot A+\cos A=p$ and $\cot A-\cos A=q$, then which of the following relation is correct?
Option 1: $\sqrt{\frac{1}{16pq}}=p^2+q^2$
Option 2: $\frac{1}{4pq}=p^2+q^2$
Option 3: $\sqrt{16pq}=p^2-q^2$
Option 4: $4pq=p^2+q^2$
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Correct Answer: $\sqrt{16pq}=p^2-q^2$
Solution :
Given: $\cot A+\cos A=p$ and $\cot A-\cos A=q$
$p^{2}=\cot^{2} A+\cos^{2} A+2\cot A.\cos A$
$q^{2}=\cot^{2} A+\cos^{2} A-2\cot A.\cos A$
⇒ $p^{2}–q^{2}=4\cot A.\cos A$
Also, $pq=\cot^{2} A-\cos^{2} A$
$=\frac{\cos^{2} A–cos^{2}A.\sin^{2} A}{\sin^{2} A}$
$=\frac{\cos^{2} A(1–\sin^{2} A)}{\sin^{2} A}$
$=\cot^{2} A(1–\sin^{2} A)$
$=\cot^{2} A\cos^{2} A$
$=(\cot A\cos A)^{2}$
$\sqrt{pq}=\cot A\cos A$
Multiplying by 4, we get:
$\sqrt{16pq}=4\cot A\cos A$
$\sqrt{16pq}=p^{2}-q^{2}$
Hence, the correct answer is $\sqrt{16pq}=p^{2}-q^{2}$.
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