1. х^2-25= (х-5)(х+5).
2. 36m^2-100n^2= (6m-10n)(6m+10n).
3. (4-y)^2= 16-8y+y^2.
4. 36a^2+24ab+4b^2= (6a+2b)^2.
(x2 - 6x - 21) (x3 - 6x) + 80 = 0
x4 - 12x3 +15x2 + 126x + 80 = 0
(x-8)(x+2)(x2 - 6x - 5) = 0
x-8 = 0
x+2=0
x2 - 6x - 5 =0
x=8
x= - 2
x2 - 6x = 5 x2 - 6x + 9 = 14 (x-3)2 = 14 х = 3 + √14 или х-3= - <span>√14</span>
5 + 2х = 8 (х - 2)
5 + 2х = 8х - 16
2х - 8х = - 16 - 5
- 6х = - 21
х = - 21/6 = - 3 3/6 = - 3,5
Ответ: - 3,5