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Mathematics

When 2x3 - x2 - 3x + 5 is divided by 2x + 1, then the remainder is

  1. 6
  2. -6
  3. -3
  4. 0

Factorisation

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Answer

By remainder theorem, on dividing f(x) by (x - a), the remainder left is f(a).

f(x) = 2x3 - x2 - 3x + 5

∴ On dividing f(x) by 2x + 1 or 2(x(12))2(x - \big(-\dfrac{1}{2}\big)), Remainder = f(12)\big(-\dfrac{1}{2}\big)

f(12)=2(12)3(12)23(12)+5=2(18)14+32+5=1414+32+5=11+6+204=244=6f\big(-\dfrac{1}{2}\big) = 2\big(-\dfrac{1}{2}\big)^3 - \big(-\dfrac{1}{2}\big)^2 - 3\big(-\dfrac{1}{2}\big) + 5 \\[1em] = 2\big(-\dfrac{1}{8}\big) - \dfrac{1}{4} + \dfrac{3}{2} + 5 \\[1em] = -\dfrac{1}{4} - \dfrac{1}{4} + \dfrac{3}{2} + 5 \\[1em] = \dfrac{-1 -1 + 6 + 20}{4} \\[1em] = \dfrac{24}{4} \\[1em] = 6

∴ Option 1, is the correct option.

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