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How does factoring and cancelling resolve the indeterminate limit of (x2+3x−10x^2+3x-10)/(x2−2xx^2-2x) as x approaches 2?

Factoring and cancelling resolves the limit by simplifying the expression to a form that is defined near the target point. For x≠2x \ne 2 near 2, the expression x2+3x−10x2−2x\frac{x^2+3x-10}{x^2-2x} factors into (x−2)(x+5)x(x−2)\frac{(x-2)(x+5)}{x(x-2)}. Cancelling the common factor (x−2)(x-2) yields x+5x\frac{x+5}{x}. The limit as x→2x \to 2 is then evaluated by direct substitution into the simplified function, resulting in 7/27/2. This technique works because cancellation concerns nearby inputs, not the evaluation of the original expression at its missing point.

Conditions

  • The limit is as x→2x \to 2.
  • The original expression is undefined at x=2x=2 (indeterminate form 0/00/0).
  • Cancellation is valid only for x≠2x \ne 2.

Reasoning, step by step

  1. Identify the indeterminate form by substituting x=2x=2 into the original expression.
  2. Factor the numerator x2+3x−10x^2+3x-10 into (x−2)(x+5)(x-2)(x+5).
  3. Factor the denominator x2−2xx^2-2x into x(x−2)x(x-2).
  4. Cancel the common factor (x−2)(x-2) from the numerator and denominator, noting that this is valid for x≠2x \ne 2.
  5. Evaluate the limit of the simplified expression x+5x\frac{x+5}{x} as x→2x \to 2 by direct substitution.
  6. Calculate the final value: 2+52=72\frac{2+5}{2} = \frac{7}{2}.

Example

The script states: "For x≠2x\ne2 near 2, x2+3x−10x2−2x=(x−2)(x+5)x(x−2)=x+5x\frac{x^2+3x-10}{x^2-2x}=\frac{(x-2)(x+5)}{x(x-2)}=\frac{x+5}{x}. Its limit as x→2x\to2 is 7/27/2."

Common misconceptions

  • Believing that cancelling factors changes the value of the function at the point x=2x=2.
  • Thinking that the limit does not exist because the original function is undefined at x=2x=2.
  • Confusing the simplified function with the original function globally, rather than just near the limit point.

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