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Suatu barisan aritmatika dengan


U_(4)+U_(8)+U_(11)+U_(17)=316.
Nilai 
U_(10) adalah
(A) 80
(B) 79
(C) 78
(D) 77
(E) 76

11. Suatu barisan aritmatika dengan\newlineU4+U8+U11+U17=316. \mathrm{U}_{4}+\mathrm{U}_{8}+\mathrm{U}_{11}+\mathrm{U}_{17}=316 . \newlineNilai U10 \mathrm{U}_{10} adalah\newline(A) 8080\newline(B) 7979\newline(C) 7878\newline(D) 7777\newline(E) 7676

Full solution

Q. 11. Suatu barisan aritmatika dengan\newlineU4+U8+U11+U17=316. \mathrm{U}_{4}+\mathrm{U}_{8}+\mathrm{U}_{11}+\mathrm{U}_{17}=316 . \newlineNilai U10 \mathrm{U}_{10} adalah\newline(A) 8080\newline(B) 7979\newline(C) 7878\newline(D) 7777\newline(E) 7676
  1. Define Terms: Let's denote the first term of the sequence as aa and the common difference as dd. The nnth term of an arithmetic sequence is given by Un=a+(n1)dU_n = a + (n-1)d.
  2. Write Given Terms: We can write the given terms using the formula for the nth term: U4=a+3dU_4 = a + 3d U8=a+7dU_8 = a + 7d U11=a+10dU_{11} = a + 10d U17=a+16dU_{17} = a + 16d
  3. Add Given Terms: Now, let's add these up as per the given equation:\newlineU4+U8+U11+U17=(a+3d)+(a+7d)+(a+10d)+(a+16d)=316U_4 + U_8 + U_11 + U_17 = (a + 3d) + (a + 7d) + (a + 10d) + (a + 16d) = 316
  4. Simplify Equation: Simplify the equation: 4a+36d=3164a + 36d = 316
  5. Introduce Another Equation: We need to find U10U_{10}, which is a+9da + 9d. But we only have one equation and two unknowns. We need another equation to solve for aa and dd.
  6. Apply Spacing Property: Notice that the terms are evenly spaced, so the sum of pairs equidistant from the ends should be equal. That means U4+U17=U8+U11.U_4 + U_{17} = U_8 + U_{11}.
  7. Simplify Equation: Let's write that out:\newline(a+3d)+(a+16d)=(a+7d)+(a+10d)(a + 3d) + (a + 16d) = (a + 7d) + (a + 10d)
  8. Subtract Terms: Simplify the equation: 2a+19d=2a+17d2a + 19d = 2a + 17d
  9. Subtract Terms: Simplify the equation:\newline2a+19d=2a+17d2a + 19d = 2a + 17d Subtract 2a2a from both sides:\newline19d=17d19d = 17d

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