Question

Дано: (аn) - арифметична прогресія; а1 = 15, d = 3. Знайти: а2 + а4 ​

Answer 1

$\displaystyle\bf\\a_{1} =15\\\\d=3\\\\a_{2} +a_{4} =a_{1} +d+a_{1} +3d=2a_{1} +4d=2\cdot 15+4\cdot3=\\\\=30+12=42\\\\Otvet: \ a_{2} +a_{4} =42$