Algebra · real student question

Solve the inequality -2|3m| + 3 < 51.

Question

Solve

23m+3<51-2|3m|+3<51

Step-by-step solution

  1. Isolate the absolute value term. Subtract 33 from both sides:

    23m<48-2|3m|<48

  2. Divide by 2-2 and reverse the sign. Division by a negative flips the direction:

    3m>24|3m|>-24

  3. Stop and read the inequality instead of splitting it. The usual split A<bA<-b or A>bA>b assumes b>0b>0. Here the right-hand side is negative, so that template does not apply. Compare the two sides directly:

    3m0>24|3m|\ge 0>-24

  4. Conclude that every real number is a solution. Since 3m|3m| is at least 00 and 00 already exceeds 24-24, the inequality holds for every mm:

    m(,)m\in(-\infty,\infty)

    Spot-check m=0m=0: 20+3=3<51-2\cdot 0+3=3<51 \checkmark; and m=1000m=1000: 6000+3=5997<51-6000+3=-5997<51 \checkmark.

  5. Compare with the near-identical problem that is not trivial. Changing the right-hand side from 5151 to 51-51 gives 3m>27|3m|>27, whose solution is only m<9m<-9 or m>9m>9. The sign of the number left after the flip is what decides between 'all reals' and a genuine two-branch answer.

Answer

m(,)(every real number)m\in(-\infty,\infty)\quad\text{(every real number)}

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