Q.If \(p+q=\sqrt{13}\) and \(p−q=\sqrt{5}\), then the value of \(pq\) is— (a) 2 (b) 18 (c) 9 (d) 8
Answer: A
\(pq=\cfrac{(p+q)^2-(p-q)^2}{4}\)
\(=\cfrac{(\sqrt{13})^2-(\sqrt5)^2}{4}\)
\(=\cfrac{13-5}{4}\)
\(=2\)
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