4,2,1,...,a=-0,004 -9,3,-1,...,a=-0,009 4,-2,1,...,a=0,01 Помогите пожалуйста Ау

B₁ = 4 b₂ = 2
q = b₂ : b₁ = 2 : 4 = 1/2
$b _{n}=a\\\\S _{n} = \frac{b _{n}q- b_{1} }{q-1}\\\\-0,004= \frac{ \frac{1}{2} a-4}{ \frac{1}{2}-1 } \\\\-0,004= \frac{0,5a-4}{-0,5}\\\\0,5a-4=(-0,004)*(-0,5)\\\\0,5a-4=0,002\\\\0,5a=4,002\\\\a=8,004$

$b _{1}=-9\\ b_{2} =3\\q= \frac{ b_{2} }{ b_{1} }= \frac{3}{-9}=- \frac{1}{3}\\ b_{n} =a\\\\S _{n}= \frac{ b_{n}q- b_{1} }{q-1} \\\\-0,009= \frac{- \frac{1}{3}a+9 }{- \frac{1}{3}-1 }\\\\-0,009= \frac{- \frac{1}{3}a+9 }{- \frac{4}{3} }\\\\- \frac{1}{3}a+9=-0,009*(- \frac{4}{3})\\\\- \frac{1}{3}a+9=0,012\\\\- \frac{1}{3}a=-8,988\\\\a=26,964$

$b_{1} =4\\ b_{2}=-2\\q= \frac{ b_{2} }{ b_{1} } = \frac{-2}{4} =- \frac{1}{2} \\ b_{n} =a\\\\S _{n}= \frac{ b_{n}q- b_{1} }{q-1}\\\\0,01= \frac{-0,5a-4}{-0,5-1}\\\\0,01= \frac{-0,5a-4}{-1,5}\\\\-0,5a-4=0,01*(-1,5) \\\\-0,5a-4=-0,015\\\\-0,5a=3,985\\\\a=-7,97$