Question

calculati primi 3 termenii a1,a2,a3,5,7,9,11..
a1,a2,a3=?
a 15
S27

Answer 1

ratia r = an-an-1 = 7-5 = 2 ;

a3 = 5-2 = 3 ; a2 = 3-2 = 1 ; a1 = 1-2 = -1 ;

a15 = a1+(n-1)*r = -1+(15-1)*2 = -1+14*2 = -1+28 = 27 ;

S27 = (a1+an)*n/2 ; calculam a27 = -1+(27-1)*2 = 51 ;

S27 = (-1+51)*27/2 = 50*27/2 = 1350/2 = 675 ;