7X + 8=11
7X=11-8
7x=3
x=3,5
4x +4=x+5
4x-x=5-4
3x=1
x=1:3
А. (m-9)(m+9)
б. (b-11c)(b+11c)
в. (13m-4n)(13m+4n)
г. (6m-kp)(6m+kp)
2cos²x-1-1-4cosx=0
2cos²x-4cosx-2=0
cos²x-2cosx-1=0
cosx=a
a²-2a-1=0
D=4+4=8
a1=(2-2√)2)/2=1-√2⇒cosx=1-√2⇒x=+_arccos(1-√2)+2πn
a1=(2+2√)2)/2=1+√2⇒cosx=1+√2∉[-1;1]
а9 = а5 + 4d = -18 + 4(-3) = -18 - 12 = -30
X≠-3
x-105=-5(x+3)
x+5x=105-15
6x=90
x=15