A5=9 a2+a9=20 Найдите S10-?

Question

A5=9
a2+a9=20
Найдите S10-?

Answer 1

a1+4d=9aa1+d+a1+8d=20
2a1+10d=20
a1=9-4d    18-8d+10d=20    2d=2    d=1   a1=9-4*1=5
a10=5+9*1=14
s10=(a1+a10)*10/2=(5+14)*5=95