Question

calculati suma primilor 10 termeni .a1 =15 ,r=3

Answer 1

$\displaystyle a_1=15,~r=3,~S_{10}=? ~~~~~~~~~~~~~~~~~~~~ \boxed{S_n= \frac{2a_1+(n-1) \cdot r}{2} \cdot n} \\ S_{10}= \frac{2 \cdot 15+(10-1) \cdot 3}{2} \cdot 10 \\ S_{10}= \frac{30+9 \cdot 3}{2} \cdot 10 \\ S_{10}= \frac{30+27}{\not2} \cdot \not10 \\ S_{10}=57 \cdot 5 \\ S_{10}=285$