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Algebra · real student question

Solve x^2 - 3x - 10 >= 0 and x^2 - 1 <= 0, and find the values of x satisfying both.

Question

Solve each inequality, then find all xx satisfying both:

x2−3x−10≥0,x2−1≤0x^{2}-3x-10\ge 0,\qquad x^{2}-1\le 0

Step-by-step solution

  1. Factor the first quadratic. Two numbers with product −10-10 and sum −3-3 are −5-5 and +2+2:

    x2−3x−10=(x−5)(x+2)≥0x^{2}-3x-10=(x-5)(x+2)\ge 0

    Critical points: x=−2x=-2 and x=5x=5.

  2. Read the first solution set off the parabola. The leading coefficient is positive, so this upward parabola is at or above the axis outside its roots (test x=0x=0: −10<0-10<0, so the middle interval fails):

    (−∞,−2]∪[5,∞)(-\infty,-2]\cup[5,\infty)

    Both endpoints are included because the inequality is non-strict.

  3. Factor the second quadratic as a difference of squares.

    x2−1=(x−1)(x+1)≤0x^{2}-1=(x-1)(x+1)\le 0

    Critical points: x=−1x=-1 and x=1x=1.

  4. Read the second solution set. An upward parabola is at or below the axis between its roots (test x=0x=0: −1≤0-1\le 0 ✓):

    [−1,1][-1,1]

  5. Intersect the two sets. Overlay them on one number line: the second set lives entirely inside (−2,5)(-2,5), which is precisely the interval the first set excludes. Formally [−1,1]∩(−∞,−2]=∅[-1,1]\cap(-\infty,-2]=\varnothing and [−1,1]∩[5,∞)=∅[-1,1]\cap[5,\infty)=\varnothing, so

    ((−∞,−2]∪[5,∞))∩[−1,1]=∅\left((-\infty,-2]\cup[5,\infty)\right)\cap[-1,1]=\varnothing

  6. State both answers. Individually: (−∞,−2]∪[5,∞)(-\infty,-2]\cup[5,\infty) and [−1,1][-1,1]. Together: no real xx satisfies both — the system is inconsistent, which is worth checking whenever two quadratic inequalities are imposed at once.

Answer

x2−3x−10≥0: (−∞,−2]∪[5,∞);x2−1≤0: [−1,1];both: ∅x^{2}-3x-10\ge 0:\ (-\infty,-2]\cup[5,\infty);\quad x^{2}-1\le 0:\ [-1,1];\quad \text{both: } \varnothing

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