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Jika diketahui U adalah suku ke-n suatu deret . aritmatika dan u1 + u2 + u3 = -9; u3 + u 4 + u5 = 15, maka ·jumlah 5 suku pertama deret tersebut
adalah ....

  1. 4

  2. 5

  3. 9

  4. 15

  5. 24

A. C

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Pembahasan

u subscript 1 space plus space u subscript 2 space plus space U subscript 3 space equals space minus 9 space  a space plus space left parenthesis a space plus space b right parenthesis space plus space left parenthesis a space plus space 2 b right parenthesis space equals space minus 9  3 a space plus space 3 b space equals space minus 9  a plus space b space equals space minus 3 space  a equals space minus 3 space minus space b    U subscript 3 space plus space u space subscript 4 space plus space U subscript 5 space equals space minus 9 space  left parenthesis a plus space 2 b right parenthesis space plus left parenthesis a plus space 3 b right parenthesis space plus left parenthesis a plus space 4 b right parenthesis space equals space 15  3 a space plus space 9 b space equals space l 5  a plus space 3 b space equals space 5  left parenthesis negative 3 space minus space b right parenthesis space plus space 3 b space equals space 5  2 b space equals space 8 space  b equals 4  a equals space minus 3 space minus space b space equals space minus 3 space minus space 4 space equals space minus 7    S n space equals space n over 2 left parenthesis 2 a space plus space left parenthesis n space minus 1 right parenthesis b right parenthesis  S subscript 5 space equals space 5 over 2 left parenthesis 2 left parenthesis negative 7 space plus space left parenthesis 5 space minus 1 right parenthesis 4 right parenthesis  S subscript 5 space equals space 5 over 2 left parenthesis negative 14 plus 16 right parenthesis  S subscript 5 space equals 5

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Jumlah dari deret aritmetika 17+21+25+29+33+...+577 adalah ....

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