Suatu Suku memiliki rumus sebagai berikut: Un:N²+n-4 Tentukan A. U15= B. S 12 =

<p>a. U_n = n² + n - 4</p><p>&nbsp; &nbsp; &nbsp;U₁₅= 15² + 15 - 4</p><p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;= 225 + 15 - 4</p><p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;= 240 - 4</p><p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;= 236</p><p>b. Mencari S₁₂</p><p>&nbsp; &nbsp; &nbsp;U_n = n² + n - 4</p><p>&nbsp; &nbsp; &nbsp; U₁ = 1² + 1 - 4</p><p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;= -2</p><p>&nbsp; &nbsp; &nbsp; &nbsp;U₂ = 2² + 2 - 4</p><p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;= 4 + 2 - 4</p><p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;= 2</p><p>&nbsp; &nbsp; &nbsp; &nbsp;b = 2 - (-2) = 2 + 2 = 4</p><p>&nbsp; &nbsp; &nbsp; &nbsp; n = 12</p><p>&nbsp; &nbsp; &nbsp; &nbsp; S_n = n/2 (2a + (n - 1)b)</p><p>&nbsp; &nbsp; &nbsp; &nbsp; S₁₂ = 12/2 (2(-2) + (12 - 1)4)</p><p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; = 6 (-4 + (11)4)</p><p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; = 6 (-4 + 44)</p><p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;= 6 (40)</p><p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;= 240</p><p>&nbsp; &nbsp; &nbsp; Jadi, S₁₂ adalah 240</p>