Арифметическая прогрессия содержит члены a7=54 и a15=110, найдите a3.
15-7=8
a15=a7+8d
110=54+8d
8d=110-54
8d=56
d=7
a7=a1+6d
54=a1+6*7
a1=54-42=12
a3=a1+2d=12+2*7=26
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15-7=8
a15=a7+8d
110=54+8d
8d=110-54
8d=56
d=7
a7=a1+6d
54=a1+6*7
a1=54-42=12
a3=a1+2d=12+2*7=26