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Evaluating a polynomial limit using limit laws

Consider the polynomial function f(x)=3x2−5x+2f(x) = 3x^2 - 5x + 2.

Use the limit laws (sum, difference, product, and power rules) to evaluate lim⁡x→2(3x2−5x+2).\lim_{x \to 2} \left(3x^2 - 5x + 2\right). State the value of the limit.

Hint

Since f(x)f(x) is a polynomial, it is continuous everywhere, so you can break the limit into a sum of simpler limits using the limit laws and evaluate each piece, or simply substitute x=2x = 2 directly.

Step-by-step solution

Because f(x)=3x2−5x+2f(x) = 3x^2 - 5x + 2 is a polynomial, it is continuous at every real number, including x=2x = 2. Using the sum and difference rules, lim⁡x→2f(x)=3lim⁡x→2x2−5lim⁡x→2x+lim⁡x→22\lim_{x\to 2} f(x) = 3\lim_{x\to2}x^2 - 5\lim_{x\to2}x + \lim_{x\to2}2. The power rule gives lim⁡x→2x2=(lim⁡x→2x)2=22=4\lim_{x\to2}x^2 = \left(\lim_{x\to2}x\right)^2 = 2^2 = 4, and lim⁡x→2x=2\lim_{x\to2}x = 2.

Substituting these values in: 3(4)−5(2)+2=12−10+2=43(4) - 5(2) + 2 = 12 - 10 + 2 = 4. This matches what direct substitution of x=2x=2 into f(x)f(x) would give, confirming the result. Therefore lim⁡x→2(3x2−5x+2)=4\lim_{x\to2}(3x^2 - 5x + 2) = 4.

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