Question

an арифметитеческая прогрессия a14=140 S14=1050 a1=? d=?

Answer 1

а14=a1+13d=140  
s14=1/2(a1+a14)*14=1/2(2a1+13d)*14=1050

a1=140-13d    7(280-26d+13d)=7(280-13d)=1050   280-13d=150
13d=280-150=130  d=10  
a1=140-130=10