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an=a1+(n-1)r
czyli:
a4=a1+3r
a7=a1+6r
powstaje układ z 2 niewiadomymi a1 i r
a1+3r=3
a1+6r=9/*(-1)
a1+3r=3
-a1-6r=-9 (dodaje stronami)
-----------------
-3r=-6
r=2
a1=-3
a30=a1+29r=-3+29*2=-3+58=55
2a1+(n-1)r
Sn= ---------- *n
2
S30=2*-3+29*2
---------- * 30
2
S30=-6+58r*30/2
=-3+29*30=867
prosze ;]