b₁ = - 25 b₂ = - 20 b₄ = ?
b₂ = b₁ * q
q = b₂ : b₁ = - 20 : (- 25) = 4/5
b₄ = b₁ * q³ = - 25 * (4/5)³ = - 5² * (4/5)³ = - 64/5 = - 12,8
2x-1=10x+3
2x-10x=3+1
-8x=4|:(-8)
x=-4/8
x=-1/2
x=-0,5.