Verified answer

[tex]\displaystyle\bf\\a_{10}=20\\\\d=-1\\\\a_{1} =?\\\\\\a_{10}=a_{1} +9d\\\\a_{1} =a_{10} -9d=20-9\cdot(-1)=20+9=29\\\\Otvet \ : \ a_{1} =29[/tex]

Ответ:

а1=29

Объяснение:

решение внизу