1) (12x^2-5) + (3x^2+7)= =12x^2-5+3x^2+7=15x^2+2
2) непонятно
3)(8-а)-(2а-17)+6а=8-а-2а+17+6а=3а+25
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5/9х-16=3/4х+5
5/9х-3/4=5+16
-7/36х=21
х=-21*36/7=-108
у=5/9*(-108)-16=-60-16=-76
(-108;-76)
-76-108р=0
108р=-76
р=-76/108=-19/27
3x - 4x - 4 < 8 + 5x
3x - 4x - 5x < 8 + 4
- 6x < 12
x > - 2
x ∈ ( - 2; ∞)