Question

fie progresia aritmetica A (n) A (4)-A (2)=4 si A (1)+ A (3) + A (5)+ A (6)=30
sa se afle suma primilor 20 termeni !!!

Answer 1

[tex] a_{4}- a_{2}=4 [/tex]\
$a_{1}+ a_{3}+ a_{5}+ a_{6} =30$
$a_{1}+3r- a_{1}+r=4$ si $a_{1}+ a_{1}+2r+ a_{1}+4r+ a_{1}+5r=30$
din prima relatie 4r=4⇒r=1
inlocuim in relatia a doua si obtinem 4$a_{1}+11=30$
4a1=19⇒a1=19/4