Question

даю 30 баллов помогите срочно пжпжпжпж

Answer 1

$a_{7} = 6 \\ a_{17} = 26 \\ a_{n} = a_{1} + (n - 1)d \\ \\ a_{7} =a _{1} + 6d \\ a_{17} = a_{1} + 16d \\ \\ a _{1} + 6d = 6 \: \: | \times ( - 1)\\ a_{1} + 16d = 26 \\ \\ - a _{1} - 6d = - 6 \\ a_{1} + 16d = 26 \\ \\ - a _{1} - 6d + a_{1} + 16d = 26 - 6 \\ 10d = 20 \\ d = 20 \div 10 \\ d = 2 \\ \\ a_{1} + 6 \times 2 = 6 \\ a_{1} + 12= 6 \\ a_{1} = 6 - 12 \\ a_{1} = - 6 \\ \\ S_{n} = \frac{2a_{1} + (n - 1)d}{2} n \\ S_{40} = \frac{2a_{1} + 39d}{2} \times 40 = \\ =( 2 \times ( - 6) + 39 \times 2) \times 20 = \\ = ( - 12 + 78) \times 20 = 66 \times 20 = 1320$