1. an=n²-3n a9=81-27=54
2. a5=-4+6*4=20
3. a1+2d=8 a1+6d=-4 -4d=12 d=-3
4. a1=-3 d=5 s=(-2*3+11*5)*12/2=49*6=294
5. a1+3d=26 a1+7d=68 4d=68-26=42 d=10.5
a1+3*10.5=26 a1=26-31.5=-5.5 a21=-5.5+20*10.5=-5.5+ 210=204.5
6. 6;4.8;3.6 a1=6 d=4.8-6=-1.2 an=6-1.2(n-1)=6-1.2n+1.2=7.2-1.2n
7.2-1.2n>0 1.2n<7.2 n = 6
-2x=4-x x=-4 y=8
-2x=1/3x x=0 y=0
1/3x=4-x x=3 y=1
ответ (-4,8) (0,0) (3,1)