Дана возрастающая арифметическая прогрессия а1=3,а2=7,а3=11,... найдите значения...

D = a2 - a1 = 7 - 3 = 4
$sn= \frac{a1 + an}{2} \times n = \frac{2a1 + d(n - 1)}{2} \times n \\ s5 = \frac{2a1 + 4d}{2} \times 5 = \frac{2 \times 3 + 4 \times 4}{2} \times 5 = 11 \times 5 = 55$
$sn= \frac{a1 + an}{2} \times n = \frac{2a1 + d(n - 1)}{2} \times n \\ s5 = \frac{2a1 + 4d}{2} \times 5 = \frac{2 \times 3 + 4 \times 4}{2} \times 5 = 11 \times 5 = 55 \\ s7 = \frac{2a1 + 6d}{2} \times 7 = \frac{2 \times 3 + 6 \times4 }{2} \times 7 = 15 \times 7 = 105 \\ s7 \times 3 \times s5 = 105 \times 3 \times 55 = 17325$