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a1 = 1
an+1 = an + n² dla n > 1
zatem mamy
a2 = a1 + 1² = 1 + 1 = 2
a3 = a2 + 2² = 2 + 4 = 6
a4 = a3 + 3² = 6 + 9 = 15
a5 = a4 + 4² = 15 + 16 = 31
a6 = a5 + 5² = 31 + 25 = 56
a7 = a6 + 6² = 56 + 36 = 92
czyli
2*a5 - a7 = 2* 31 - 92 = 62 - 92 = - 30
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