Решение:
\ a_1=a_4-3d=23-12=11.\\ a_{18}=a_1+17d=11+17\cdot 4=79.\\ \\ Answer: \ 79." alt="a_4=23.\\ d=4.\\ a_1_8 - ? \\\\ Decision:\\\\ a_4=a_1+3d; \ =>\ a_1=a_4-3d=23-12=11.\\ a_{18}=a_1+17d=11+17\cdot 4=79.\\ \\ Answer: \ 79." align="absmiddle" class="latex-formula">