[tex]\displaystyle f(x)=4x-1 \\ f(1)+f(2)+...+f(28)=? \\ -------------\\ f(1)=4\cdot1-1=4-1=3 \\f(2)=4\cdot2-1=8-1=7 \\f(3)=4\cdot3-1=12-1=11 \\ f(4)=4\cdot4-1=16-1=15\\ f(5)=4\cdot5-1=20-1=19\\ f(6)=4\cdot6-1=24-1=23 \\.\\.\\. \\ f(27)=4\cdot27-1=107 \\ f(28)=4\cdot 28-1=111 \\ \\\text{Observam ca rezultatele au ca ratie 4, deci este o progresie aritmetica} \\ \\ S= \frac{(3+111) \cdot 28 }{2} = \frac{114 \cdot \not28}{\not2} = 114 \cdot 14=1596 \\ \\ Formula \ folosita \ la \ final \ este [/tex]
