Ответ:
a3=a1+2d. a12=a1+11d. a1=a3-2d, a1=a12-11d, 18-2d=54-11d, 9d=36, d=4.
a1=18-(2×4)=10. a14=10+(13×4)=62
S14=(10+62)×14/2=504
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Ответ:
a3=a1+2d. a12=a1+11d. a1=a3-2d, a1=a12-11d, 18-2d=54-11d, 9d=36, d=4.
a1=18-(2×4)=10. a14=10+(13×4)=62
S14=(10+62)×14/2=504