Suatu barisan aritmatika mempunyai u6=23 dan u10=43.jumlah 15 suku pertama adalah

U6 = a + 5b = 23
U10 = a + 9b = 43 ...........(eliminasi)
                   -4b = -20
                       b = 5
a + 5b = 23
a + 25 = 23
        a = -2
U15 = a+14b
        = -2 + 14(5)
        = -2 + 70
        = 68
S15 = 1/2 . n (a + U15)
        = 15/2 ( -2 + 68)
        = 15/2 (66)
        = 15 . 33
        = 495 

Diket : U6 = 23 dan U10 = 43

=》 cari 'b',
Beda = 43-23/10-6
Beda = 20/4
Beda = 5
=》 cari 'a'
U6 = a+5b
23 = a+5.5
23 = a+25
a = -2
=》 U15 = a+14b => -2+14.5=>68
=》 S15 = n/2(a+Un)
S15 = 15/2(-2+68)
S15 = 15/2(66)
S15 = 495

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