2. 21x² = 5x-1
21х²-5х+1 = 0
D = 25-84 = -59
D<0 - корней нет<br>
3. 8х²+х-9 = 0
D = 1+288 = 289
x₁ = -1-17 / 16 = -18/16 = -9/8
х₂ = -1+17 / 16 = 16/16 = 1
4. (х-3)²-2(х-3)-15 = 0
х²-6х+9-2х+6-15 = 0
х²-8х = 0
х(х-8) = 0
х₁ = 0 или х-8=0
х₂ = 8
5. х²/3 + 13х/12 - 1 = 0
4х²+13х-12 = 0
D = 169+192 = 361
х₁ = -13-19 / 8 = -32/8 = -4
х₂ = -13+19 / 8 = 6/8 = 3/4 = 0,75