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a₁
a₂=a₁+3
a₃=a₁+6
a₄=a₁+9
a₅=a₁+12
a₁+a₃+a₅=21
a₁+a₁+6+a₁+9=21
3a₁+15=21
3a₁=6 /÷3
a₁=2, czyli a₂=5, a₃=8, a₄=11, a₅=14
S=a₁+a₂+a₃+a₄+a₅=2+5+8+11=14=40
a₅=a₁×q⁴
14=2×q⁴ /÷2
q⁴=7
q=7 lub q=-7