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a4=26
teraz obliczamy roznice
a4-a3=r
26-13=13
a3=a1+2r
13=a1+26 /-26
a1=-13
a1 wynosi -13
an=a₁+(n-1)r
a₃=a₁+(3-1)r=a₁+2r
a₅=a₁+(5-1)r=a₁+4r
czyli mamy układ równań:
a₁+2r=13
a₁+4r=39
a₁=13-2r
13-2r+4r=39
a₁=13-2r
2r=39-13
a₁=13-2r
2r=26 /:2
a₁=3-2r
r=13
a₁=13-2*13=13-26=-13
Odp. a₁=-13