Bonsoir,
[tex]\textnormal{Soit $(u_n)$ une suite arithm\'etique :}[/tex]
[tex]u_1= u_0 + n \times r \iff 21 + 1 \times (-4) = 17[/tex]
[tex]u_2 = u_0 + n \times r \iff 21 + 2 \times (-4) = 21 - 8 = 13[/tex]
[tex]u_3 = u_0 + n \times r \iff 21 + 3 \times (-4) = 21 - 12 = 9[/tex]
[tex]u_4 = u_0 + n \times r \iff 21 + 4 \times (-4) = 21 - 16 = 5[/tex]