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Q4. The sum of reciprocals of Rehman’s ages (in years) 3 years ago and 5 years from now is 13. Find his present age.
Answer :

Let present age of Rehman = x years
Age of Rehman 3 years ago = (x - 3) years.
Age of Rehman after 5 years = (x + 5) years
According to the given condition:
\({{1} \over {x - 3}} + {{1} \over {x + 5}} = {{1} \over {3}}\)
=>\({{(x + 5) + (x - 3)} \over {(x - 3)(x + 5)}} = {{1} \over {3}}\)
=>\( 3 (2x + 2) = (x - 3) (x + 5)\)
=>\( 6x + 6 = x^2 - 3x + 5x -15\)
=>\(x^2 - 4x - 15 - 6 = 0\)
=>\(x^2 - 4x -21 = 0\)
Comparing quadratic equation \(x^2 - 4x -21 = 0\) with general form \(ax^2 + bx + c = 0\),
We get a = 1, b = ?4 and c = ?21
Using quadratic formula = \(x = {{-b ± \sqrt{b^2 - 4ac}} \over {2a}}\)
=>\(x = {{4 ± \sqrt{(4)^2 - 4(1)(-21)}} \over {(2)(1)}}\)
=>\(x = {{4 ± \sqrt{16 + 84}} \over {2}}\)
=>\(x = {{4 + 10} \over {2}} , {{4 - 10} \over {2}}\)
=>\(x = 7, -3\)
We discard x=-3. Since age cannot be in negative.
Therefore, present age of Rehman is 7 years.