Question

calculați progresia aritmetica
a1=3
r=5
________
a7,S10
Urgent

Answer 1

$\displaystyle\bf\\Se~da~progresia~aritmetica:\\a_1=3\\r=5\\Se~cere:\\a_7=?\\S_{10}=?\\\\Rezolvare:\\a_7=a_1+(7-1)\times r=3+6\times5=3+30=33\\\boxed{\bf~a_7=33}\\\\a_{10}=a_1+(10-1)\times r=3+9\times5=3+45=48\\\boxed{\bf~a_{10}=48}\\\\S_{10}=\frac{10(48+3)}{2}=5\times51=255\\\boxed{\bf~S_{10}=255}$