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a3=a1+2r
a7=a1+6*r
a7=a3+(7-3)*r
a7=a3+4r
12=(-5)+4*r
4r=12+5
4r=17/:4
r=17/4=4+1/4
a3=a1+2*(4+1/4)
(-5)=a1+8+1/2
a1=-(8+1/2)+(-5)
a1=-13+1/2
a10=(-13+1/2)+9*(4+1/4)
a10=(-13+1/2)+(38+1/4)=24,75
a20=(-13+1/2)+19*(4+1/4)=(-13+1/2)+(80+3/4)=67,25
ciąg jest rosnący, bo a1=-13,5 ; a3=-5 ; a7=12 ; a10=24,75 ; a20=67,25