Obliczyć sumę siedmiu początkowych wyrazów ciągu arytmetycznego, w którym a3=7 i a13-a9=20
a₁₃ - a₉ = 20
a₁₃ = 20 + a₉
a₉ + 4r = 20 + a₉
4r = 20
r = 5
a₁ + 2r = 7
a₁ + 10 = 7
a₁ = -3
a₇ = a₁ + 6r
a₇ = -3 + 30 = 27
S₇ = (-3 + 27)*7/2 = 24 * 7 / 2 = 84
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a₁₃ - a₉ = 20
a₁₃ = 20 + a₉
a₉ + 4r = 20 + a₉
4r = 20
r = 5
a₁ + 2r = 7
a₁ + 10 = 7
a₁ = -3
a₇ = a₁ + 6r
a₇ = -3 + 30 = 27
S₇ = (-3 + 27)*7/2 = 24 * 7 / 2 = 84