f(1) = -3×1 + 7 = - 3 + 7 = 4
f(2) = -3×(2) + 7= - 6 + 7 = 1
f(3) = -3×(3) + 7= - 9 + 7 = - 2
f(4) = -3×(4) + 7= - 12 + 7 = - 5
f(37)=-3×(37)+7= -111 + 7 = - 104
S= f(1) + f(2)+....+f(37)= 4 + 1 + (-2) + (-5) = 4 + 1 - 2 + - 5 ........- 104 =
suma este progresie aritmetica cu ratia -3
an=a1+(n-1)×r cu a1=4 si an=-104
an=a1+(n-1)×r
-104=4+(n-1)×(-3)
-104=4 - 3n+3
3n=104+4+3
3n=111
n=111/3
Sn=[(a1+an)×n]:2=[(4-104)×111/3]:2==(-100×111/3):2=-11100/3×1/2=-1850
