Question

Suku ke-3 dan suku ke-7 barisan aritmatika berturut turut 10 dan 22. jumlah suku ke 30 suku pertama barisan tersebut?

Answer 1

U3 =  a + 2b
10 = a + 2b

U7 = a + 6b
22 = a + 6b

eliminasikan
22 = a + 6b
10 = a + 2b    -
12 = 4b
12 : 4 = b
3 = b

cari suku awal
a + 2b = 10
a + 2(3) = 10
a + 6 = 10
a = 10-6
a = 4

maka a = 4, b = 3
Sn = n/2 (2a + (n-1)b)
S30 = 30/2 [(2 x 4) + (30-1)3]
S30 = 15 [8 + (29 x 3)]
S30 = 15 [8 + 87]
S30 = 15(95)
S30 = 1425

maka jawabannya 1425

Answer 2

B = U7 - U3 : 7 - 3 =22 - 10 : 4 = 12 : 4 = 3..skrang kta cari a nya....dngan cara subtitusikan b = 3 ke U3 d dpt U3 = 10 maka a + 2b = 10 shingga a + 2(3) = 10 d dpt a = 10 -6 = 4 mka S30 = n/2 (2a + (n - 1)b) mka 30/2 (2(4) + 29(3) = 15(8 + 87) mka 15(95) = 1425