Question

a1=2, an=87 ,sn=801. Найдите d и n помогите пожалуйста

Answer 1

$a_{1}=2\\\\a_{n}=87\\\\S_{n}=801\\\\d=?\\\\n=?\\\\S_{n}=\frac{a_{1}+a_{n}}{2}*n\\\\801=\frac{2+87}{2}*n\\\\44,5n=801\\\\n=18\\\\a_{18}=a_{1}+17d\\\\17d=a_{18} -a_{1}=87-2=85\\\\d=\frac{85}{17}=5$