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a4+a9=-22
--------------
a1+2r+a1+6r=-10
a1+3r+a1+8r=-22
--------------------
2a1+8r=-10
2a1+11r=-22
---------------odjąć stronami
-3r=12/:(-3)
r=-4
podstaw
2a1+8r=-10
2a1+8*(-4)=-10
2a1-32=-10
2a1=22
a1=11
odp a1=11 r=-4