Hello,
[tex]
u_{4}= 12+(4-4)*\frac{1}{4}\\
u_{3}= 12+(3-4)*\frac{1}{4} \\
u_{2}= 12+(2-4)*\frac{1}{4} \\
u_{1}= 12+ (1-4)*\frac{1}{4} \\
u_{5}= 12+ (5-4)*\frac{1}{4} \\
u_{6}= 12+ (6-4)*\frac{1}{4} \\
...\\
\boxed{u_{n}= 12+ (n-4)*\frac{1}{4}} \\
[/tex]
