Question

Дано: S27=60
S5=27
Найдите:d и a1
Арифметическая прогрессия!

Answer 1

S27=(2a1+26d)*27/2=60

S5=(2a1+4d)*5/2=27

(2a1+26d)*27=120

(2a1+4d)*5=54

2a1+26d=120/27

2a1+4d=54/5

1уравн отнимаем 2е ур

26d-4d=120/27 - 54/5

22d=600/135 - 1458/135

22d=-858/135

d=-858/135÷22

d=-858/135*1/22

d=-39/135

d=-13/45

2*a1 +4*(-13/45)=54/5

2a1-52/45=54/5

2a1=54/5 +52/45

2a1=486/45 + 52/45

2a1=538/45

a1=538/45 :2

a1=538/45 :1/2

a1=269/45

a1=5цел 44/45