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. (4, 2) .
3 × 4 − 2 = 12 − 2 = 10 > 5
[Quadratic Inequalities] .
(.)
(ax2 + bx + c ≤ 0 ) ax2 + bx + c < 0
(ax2 + bx + c ≥ 0 ) ax2 + bx + c > 0
ax2 + bx + c .
.x 2 < 2x + 3 ()
x 2 < 2x + 3 ⇔ x 2 − 2x − 3 < 0 ⇔ (x + 1)(x − 3) < 0
x +1 −−− −1 3
x −3 −−− +++ +++
−−− +++
