S₄=-80 S₃=-23 b₁=-2 q=?
S₄-S₃=(b₁+b₁q+b₁q²+b₁q³)-(b₁+b₁q+b₁q₂)=-80-(-26)
b₁+b₁q+b₁q²+b₁q³-b₁-b₁q-b₁q=-54
b₁q³=-54
(-2)*q³=-54 |÷(-2)
q³=27
q³=3³
q=3.
0,(6)
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1.f(-1) = 5(-1)2 - 3(-1) -1 = -10+3-1= -8
2.f(0) = 5(о)2 - 3(0) -1 = 2
3.f(2) = 5(2)2 - 3(2) - 1 = 20 - 6 - 1 = 13 , наверное так