A1+a1+d -[(a1+a1+3d)*4/2]+a1+d =
= 3a1+2d-4a1-6d=-a1-4d=14 a1=-4d-14
(a1+a1+2d)*3/2+a1+2d = 3a1+3d+a1+2d=
4a1+5d=17 4(-4d-14)+5d=-16d-56+5d=17
-11d=73 d=-73/11= -6 7/11
a1=4*73/11-14 =292/11-14= 26 6/11-14=12 6/11
{21x+14-21x-6-2x>0⇒-2x>-8⇒x<4
{(x-5)(x+8)<0 x=5 x=-8 ⇒-8<x<5
x∈(-8;4)
3a+b=x, 3a-b=y
x²-y²=(x-y)(x+y)=
=(3a+b-(3a-b)(3a+b+3a-b)=
=(3a+b-3a+b)(6a)=2b*6a
12ab+9a²b²-6ab+1=
=9a²b²+6ab+1=(3ab+1)²