Let r be a root of the equation π‘₯^2 + 2π‘₯ + 6 = 0. Then the value of the…

2022

Let r be a root of the equation π‘₯^2 + 2π‘₯ + 6 = 0.

Then the value of the expression (π‘Ÿ + 2)(π‘Ÿ + 3)(π‘Ÿ + 4)(π‘Ÿ + 5) is

  1. A.

    51

  2. B.

    -51

  3. C.

    126

  4. D.

    -126

Attempted by 32 students.

Show answer & explanation

Correct answer: D

We need the value of (r + 2)(r + 3)(r + 4)(r + 5) where r satisfies r^2 + 2r + 6 = 0.

  • Use the equation to express r^2 in terms of r: r^2 = -2r - 6.

  • Pair the factors: (r + 2)(r + 5) = r^2 + 7r + 10 = (βˆ’2r βˆ’ 6) + 7r + 10 = 5r + 4.

  • Similarly, (r + 3)(r + 4) = r^2 + 7r + 12 = (βˆ’2r βˆ’ 6) + 7r + 12 = 5r + 6.

  • Multiply the two results: (5r + 4)(5r + 6) = 25r^2 + 50r + 24. Substitute r^2 = βˆ’2r βˆ’ 6:

  • 25(βˆ’2r βˆ’ 6) + 50r + 24 = βˆ’50r βˆ’ 150 + 50r + 24 = βˆ’126.

Final answer: -126

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