Calculus · real student question

Find the value of each limit: the limit of 25 as x approaches 1, of 3x^2 as x approaches 10, of 2x^4 - x^2 as x approaches 3, of (x^3 + 2)^6 as x approaches 1, and of |-15x + 4| as x approaches 3.

Question

Find the value of each of the following limits.

  1. limx125\displaystyle\lim_{x\to 1} 25
  2. limx103x2\displaystyle\lim_{x\to 10} 3x^2
  3. limx3(2x4x2)\displaystyle\lim_{x\to 3} (2x^4 - x^2)
  4. limx1(x3+2)6\displaystyle\lim_{x\to 1} (x^3 + 2)^6
  5. limx315x+4\displaystyle\lim_{x\to 3} |-15x + 4|

Step-by-step solution

  1. Check for continuity before doing anything else. Direct substitution is valid exactly when the function is continuous at the target point. Polynomials are continuous everywhere, absolute values of polynomials are continuous everywhere, and constants are trivially continuous — so all five limits here are simply function values. There is no 0/00/0 to resolve.

  2. Limit 1: a constant function. f(x)=25f(x) = 25 never changes, so approaching x=1x = 1 changes nothing:

    limx125=25\lim_{x\to 1} 25 = 25

    The presence of xx in the limit notation does not mean xx appears in the function.

  3. Limits 2 and 3: polynomials. Substitute and evaluate carefully, respecting the exponents:

    limx103x2=3(10)2=3(100)=300\lim_{x\to 10} 3x^2 = 3(10)^2 = 3(100) = 300

    limx3(2x4x2)=2(81)9=1629=153\lim_{x\to 3} (2x^4 - x^2) = 2(81) - 9 = 162 - 9 = 153

    Note 3(10)2=3003(10)^2 = 300, not (310)2=900(3\cdot 10)^2 = 900 — the coefficient is not squared.

  4. Limit 4: a power of a polynomial. Evaluate the inside first, then raise:

    limx1(x3+2)6=(1+2)6=36=729\lim_{x\to 1} (x^3+2)^6 = (1+2)^6 = 3^6 = 729

    This uses the composition law for limits, valid because the outer function u6u^6 is continuous.

  5. Limit 5: an absolute value. Evaluate the inside, then take the absolute value:

    limx315x+4=45+4=41=41\lim_{x\to 3} |-15x+4| = |-45+4| = |-41| = 41

    The result is +41+41: an absolute value is never negative. Absolute value functions have corners, but a corner does not break continuity, so substitution is still legitimate — and in any case the corner is at x=415x = \tfrac{4}{15}, nowhere near x=3x = 3.

  6. Collect and verify. The five values are 2525, 300300, 153153, 729729 and 4141. Each was reconfirmed with a symbolic limit engine, and each equals the plain function value at the target point, as continuity guarantees.

Answer

25,300,153,729,4125,\quad 300,\quad 153,\quad 729,\quad 41

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