Question

Calculeaza 33+36+39+...150 =

Answer 1

33+36+...+150=
33+3*0+33+3*1+33+3*2+....+33+3*39=
33*40+3(1+2+...+39)=
1320+3*39*40/2=
1320+2340=3660

Answer 2

33+36+39+...150=............an=150, a1=33

an=a1+(n-1)×r

150=33+(n-1)×3

150=33+3n-3

150-33+3=3n⇒120=3n⇒n=40

S=[(a1+an)/2]×n= iar   an=150, a1=33

S=[(33+150)/2]×40⇒S=183×20⇒S=3660