Question

12+15+18+.....+300=
dau 44 de pct

Answer 1

12+15+18+....300=
15-12=3
(300-12):3+1=97 numere
mijlocul sirului 12+3*48=156
S=12+15+18+....300=12+300+ 15+ 297+....+156 = 48*312+159=15132

Answer 2

$\displaystyle 12+15+18+...+300 \\ \\ 300=12+(n-1) \times 3 \\ \\ 300=12+3n-3 \\ \\ 3n=300-12+3 \\ \\ 3n=291 \\ \\ n= \frac{291}{3} \\ \\ n=97$

$\displaystyle S_{97}= \frac{24+96 \times 3}{2} \times 97 \\ \\ S_{97}= \frac{24+288}{2} \times 97 \\ \\ S_{97}= \frac{312}{2} \times 97 \\ \\ S_{97}=156 \times 97 \\ \\ S_{97}=15132$