1) c+9d=0.4+5.4=5.8
2)a-b=0.12-1.28=1.16
3)2x+10y=-3/2+0.25
4)8c-b=-2-21/2
(а+в) (а-2в) +(2в-а) (2в+а)=а^2-2ab+ba-2b^2+4b^2-a^2=-ab+2b^2=b(2b-a)
(а+4)^2-2а*(а-3) = а^2+16+8а-2а^2+6а = -а^2+14а+16
cos(x-8)=1/8
x-8=-+/-arccos1/8+2Пn
х=+/-arccos1/8+2Пn+8, n принадлежит Z