1) 8x² +2x>21
8x² +2x-21>0
8x²+2x-21=0
D=4 +672=676=26²
x₁=(-2-26)/16= -28/16= -1.75
x₂=(-2+26)/16=24/16=1.5
+ - +
--------- -1.75 ------------- 1.5 --------------
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x∈(-∞; -1.75)U(1.5; +∞)
б) 12x-5>4x²
-4x²+12x-5>0
4x²-12x+5<0<br> 4x²-12x+5=0
D=144-80=64
x₁=(12-8)/8=4/8=0.5
x₂=(12+8)/8=20/8=2.5
+ - +
------- 0.5 ------------- 2.5 -----------
\\\\\\\\\\\\\\\\\
x∈(0.5; 2.5)