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JAMB 1990 · UME · Q21

Find the solution of the equation x−8x+15=0x - 8\sqrt x + 15 = 0.

Worked solution (try it first)
  1. Let u=xu = \sqrt x, so x=u2x = u^2 and the equation is u2−8u+15=0u^2 - 8u + 15 = 0.
  2. Factorise: (u−3)(u−5)=0(u - 3)(u - 5) = 0, so x=3\sqrt x = 3 or x=5\sqrt x = 5.
  3. Square: x=9x = 9 or x=25x = 25, option C.

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