Question

a1 =6 si a2=5 se calculeze S7

Answer 1

a1=6
a2=5 dar
 a2= a1+(n-1)*r deci
6+5r=5 => 5r= -1 => =-1/2
Acum , S7= (a1+a7)*nr(adica numarul de termeni)/2
Calculam a7=6+6* -1/2=> a7=6-3=> a7=3
nr= a7-a1/r+1 => nr=3-6/-1/2 +1  =>nr=7
S7=(6+3) *7/2 => s7= 31,5