Помогите решить: 1. C₁= -4; q= 3; S₉=?. 2. C₁= 1; q= -2; S₉=?. 3. X₄= 121,5; q= -3; S₅ =?

Решите задачу:

$1.\\S_{n}=\frac{c_{1}*q^{n-1}}{1-q}\\S_{9}=\frac{c_{1}*q^8}{1-q}=\frac{-4*3^8}{1-3}=\frac{-4*6561}{-2}=2*6561=13122\\2.\\S_{n}=\frac{c_{1}*q^{n-1}}{1-q}\\S_{9}=\frac{c_{1}*q^8}{1-q}=\frac{1*(-2)^8}{1-(-2)}=\frac{1*2565}{1+2}=\frac{256}{3}=85\frac{1}{3}$
$3.\\S_{n}=\frac{X_{1}*q^{n-1}}{1-q}$
$S_{5}=\frac{X_{1}*q^4}{1-q}\\X_{n}=X_{1}*q^{n-1}\\X_{4}=X_{1}*q^3=\ \textgreater \ X_{1}=\frac{X_{4}}{q^3}=\frac{121,5}{(-3)^3}=\frac{121,5}{-27}=-4,5\\S_{5}=\frac{-4,5*(-3)^4}{1-(-3)}=\frac{-4,5*81}{4}=\frac{-364,5}{4}=-91,125$