Question

care este suma primilor 29 de termeni din sirul 3, 7, 11, 15?​

Answer 1

 

$\displaystyle\bf\\\text{\bf~Avem o progresie aritmetica cu:}\\primul~termen:~~a_1=3~~si~ratia~~r=4\\\implies~a_{29}=a_1+4(29-1)=3+4\times28=3+112=\boxed{\bf115}\\\\Avem~sirul:~~~3+7+11+15+...+115\\\\3+7+11+15+...+115=\frac{Numarul~de~termeni\Big(a_{29}+a_1\Big)}{2}\\\\3+7+11+15+...+115=\frac{29(115+3)}{2}=\\\\=\frac{29\times118}{2}=29\times59=\boxed{\bf1711}$