A₂=10 d=-2 S=-558 n-? >0
a₂=a₁+d=a₁+(-2)=10
a₁=12
S=(2*a₁+(n-1)*d)*n/2
(2*12+(n-1)*(-2))*n/2=-558 |×2
(24-2n+2)*n=1116
(26-2n)*n=1116
26n-2n²=1116 |÷(-2)
n²-13n-558=0 D=2401 √D=49
n₁=31 n₂=-18 ∉.
Ответ: n=31.
b1=b5/q^4 S10=b1(q^10-1)/q-1
b1=-9/-81 S10=1/9(-3^10-1)/-3-1
b1 =1/9 S10=1/9*(-59050)/-4
S10=-59050/9*(-1/4)
S10=59050/36