Question

Найти а1 и а6 ,если а3=7,а7=18

Answer 1

$a3 = a1 + 2d = 7 \\ a7 = a1 + 6d = 18 \\ 4d = 11 \\ d = \frac{11}{4} \\ a1 + 2 \times \frac{11}{4} = 7 \\ a1 = \frac{14}{2} - \frac{11}{2} \\ a1 = \frac{3}{2} \\ a6 = \frac{3}{2} + 5 \times \frac{11}{4} = \frac{6 + 55}{4} = \frac{61}{4}$