Tentukan unsur unsur yang diminta . a= 15, b=-3 Sn=42 n= ...

[tex]sn \: = \frac{n}{2} (2a + (n - 1)b) \\ 42 = \frac{n}{2} (2 \times 15 + (n - 1) - 3) \\ 42 = \frac{n}{2} (30 - 3n + 3) \\ 42 = \frac{n}{2} ( - 3n + 33) \\ 84 = { - 3n}^{2} + 33n \\ 28 = { - n}^{2} + 11n \\ {n}^{2} - 11n + 28 = 0 \\ (n - 4)(n - 7) = 0 \\ n = 4 \: atau \: n = 7[/tex]