Question

Dau multe puncte!!
B)a1=5,a6=10,calculati r
H)a7=-10 si S7=-28,calculati a1,r
(Capitolul cu progresii aritmetice)

Answer 1

Răspuns

1. a1 = 5 ; a6 = 10 ; a6 = a1+(n-1)*r

10 = 5+(6-1)*r ; 10 = 5+5r ; 5r = 10-5 = 5 ; r = 5:5 = 1 ;

2. S7 = (a1+a7)*n/2 ; -28 = (a1+(-10))*7/2 ; -28 = (7a1-70)/2 ; numitor comun 2 ;

-56 = 7a1-70 ; 7a1 = 70-56 = 14 ; a1 = 14:7 = 2 ;

an = a1+(n-1)*r ; -10 = 2+6r ; 6r = -12 ; r = -12/6 = -2 ;