Question

Calculati suma primilor 20 de termeni ai sirului (an) n mai mare sau egal decat 1
1,7,13,19

Answer 1

$\it a_1=1+6\cdot0=1\\ \\ a_2=1+6\cdot1=7\\ \\ a_3=1+6\cdot2=13\\ \\ a_4=1+6\cdot3=19\\.\\.\\.\\ \\ a_{20}=1+6\cdot19=115\\ \rule{100}{0.7}\\ \\ S=20\cdot1+6\cdot(1+2+3+\ ...\ +19)=20+6\cdot\dfrac{19\cdot20}{2}=20+3\cdot19\cdot20=\\ \\ \\ =20\cdcot(1+57)=20\cdot58=1160$