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an=a₁+(n-1)r
an=a₁+nr-r
an=a₁+4n-r
a₁-r=-1
r=4 bo 4n=nr
a₁-4=-1
a₁=-1+4
a₁=3
a₅=a₁+4r
a₅=3+4razy4
a₅=3+16
a₅=19
S₅=a₁+a₅/ 2 razy5
S₅=3+19/2 razy5
S₅=22/2 razy5
S₅=11razy5
S₅=55