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Find the values of a and b if lim (x->inf) [ (x3+1)/(x2+1) - (ax+b) ] = 2.

Mathematics

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Answer

C) a = 1, b = -2\text{C) a = 1, b = -2}

Step-by-step solution

5 steps

To find the values of aa and bb, we first simplify the expression inside the limit.

  1. Combine the terms into a single fraction.

    lim⁡x→∞(x3+1x2+1−(ax+b))=2\lim_{x \to \infty} \left( \frac{x^3+1}{x^2+1} - (ax+b) \right) = 2 lim⁡x→∞(x3+1−(ax+b)(x2+1)x2+1)=2\lim_{x \to \infty} \left( \frac{x^3+1 - (ax+b)(x^2+1)}{x^2+1} \right) = 2

    Expand the product (ax+b)(x2+1)(ax+b)(x^2+1):

    (ax+b)(x2+1)=ax(x2+1)+b(x2+1)=ax3+ax+bx2+b(ax+b)(x^2+1) = ax(x^2+1) + b(x^2+1) = ax^3 + ax + bx^2 + b

    Substitute this back into the numerator:

    x3+1−(ax3+bx2+ax+b)=x3+1−ax3−bx2−ax−bx^3+1 - (ax^3 + bx^2 + ax + b) = x^3+1 - ax^3 - bx^2 - ax - b

    Rearrange the terms by powers of xx:

    (1−a)x3−bx2−ax+(1−b)(1-a)x^3 - bx^2 - ax + (1-b)

    So the limit becomes:

    lim⁡x→∞((1−a)x3−bx2−ax+(1−b)x2+1)=2\lim_{x \to \infty} \left( \frac{(1-a)x^3 - bx^2 - ax + (1-b)}{x^2+1} \right) = 2
  2. Determine the value of aa.

    For the limit of a rational function as x→∞x \to \infty to be a finite non-zero value, the degree of the numerator must be equal to the degree of the denominator.
    The degree of the denominator is 2. The current degree of the numerator is 3 (due to the x3x^3 term).
    For the limit to be finite, the coefficient of x3x^3 in the numerator must be zero.

    1−a=0  ⟹  a=11-a = 0 \implies a = 1
  3. Substitute a=1a=1 into the expression and evaluate the limit.

    With a=1a=1, the expression becomes:

    lim⁡x→∞((1−1)x3−bx2−1x+(1−b)x2+1)=lim⁡x→∞(−bx2−x+(1−b)x2+1)\lim_{x \to \infty} \left( \frac{(1-1)x^3 - bx^2 - 1x + (1-b)}{x^2+1} \right) = \lim_{x \to \infty} \left( \frac{-bx^2 - x + (1-b)}{x^2+1} \right)

    To evaluate this limit, divide every term in the numerator and denominator by the highest power of xx in the denominator, which is x2x^2:

    lim⁡x→∞(−bx2x2−xx2+1−bx2x2x2+1x2)=lim⁡x→∞(−b−1x+1−bx21+1x2)\lim_{x \to \infty} \left( \frac{\frac{-bx^2}{x^2} - \frac{x}{x^2} + \frac{1-b}{x^2}}{\frac{x^2}{x^2} + \frac{1}{x^2}} \right) = \lim_{x \to \infty} \left( \frac{-b - \frac{1}{x} + \frac{1-b}{x^2}}{1 + \frac{1}{x^2}} \right)

    As x→∞x \to \infty, the terms 1x\frac{1}{x} and 1x2\frac{1}{x^2} approach 0.

    −b−0+01+0=−b\frac{-b - 0 + 0}{1 + 0} = -b
  4. Set the result of the limit equal to 2 and solve for bb.

    −b=2  ⟹  b=−2-b = 2 \implies b = -2
  5. Compare the values of aa and bb with the given options.

    We found a=1a=1 and b=−2b=-2. This matches option (C).

    The final answer is C) a = 1, b = -2\text{C) a = 1, b = -2}.

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