A1=96, n=4, Sn=1200, найти d,n

[latex]a_1=96;n=4;S_n=1200[/latex] [latex]S_n=\frac{2a_1+(n-1)*d}{2}*n[/latex] [latex]1200=\frac{2*96+(4-1)*d}{2}*4[/latex] [latex]192+3d=600[/latex] [latex]64+d=200[/latex] [latex]d=200-64[/latex] [latex]d=136[/latex] [latex]a_n=a_1+(n-1)*d[/latex] [latex]a_4=96+(4-1)*136=504[/latex]