4-10x+6x+9-4x^2-6x-13=0
-4x^2-10x=0
4x^2+10x=0
4x(x+2,5)=0
x1=0 x2=-2,5
1) a3=a1+2d=a1+6
a1+6=10
a1=4
a8=4+3×7=25
2) a3=a1+2d=a1+2
a1+2=10
a1=8
a6=8+1×5=13
3) a11=12-4×10=-28
4) a9=a1+16
a1+16=11
a1=-5
S12=(2a1+d (n-1))/2 ×n=( -10+22)/2 ×12=72
Ответ: (-12;-5), (-5;-12)
25 - 10x + x^2 - 2,5x - x^2 = 0
-12,5x + 25 = 0
-12,5x = -25
x = 2