A sequence u1,u2,u3,… u_1, u_2, u_3, \dots\,u1,u2,u3,… is defined by
u1=3un+1=un+c,n≥1 \begin{aligned}u_1 &= 3 \\u_{n+1} &= u_n + c, \quad n \geq 1\end{aligned} u1un+1=3=un+c,n≥1Given u5=21u_5 = 21u5=21
Find the value of ccc.
Find an expression for un u_n\,un in terms of nnn.
