Question

Свойства арифметической прогресси

Answer 1

a1=28, d=1, a41=68,s41=?

sn=n(a1+an)/2

s41=41(28+68)/2=41.96/2=41.48=1968

an=a1+(n-1)d

68=28+n-1, 68=n+27, n=68-27, n=41