Soal cerita Aljabar kelas 7 SMP semester ganjil

3. Tabungan Ibu = tabungan anita
100.000 = 2x + 30.000
100.000-30.000 = 2x
70.000 = 2x
x = 35.000

100.000 = 2×35.000 + 30.000
100.000 = 70.000 + 30.000
100.000 = 100.000

1. misalkan
x = lebar
y = panjang
y = x + 3
diketahui keliling = 66, maka
K = 2x + 2y
66 = 2x + 2(x+3)
66 = 2x + 2x + 6
66 - 6 = 4x
4x = 60
x = 15

y = x + 3
y = 15 + 3
y = 18
jadi panjang = 18 
dan lebar = 15

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utk mi instan ada 30 dus, 1 dus = 144 bungkus, dan 1 bungkus = 900, maka
900 . 144 . 30
= 3.888.000
utk minyak goreng ada 50 liter, 1 liter = 5400, maka
5400 . 50
= 270.000
harganya
3.888.000 + 270.000 = 4.158.000

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Tabungan Ibu = 100.000
tabungan ibu = 2 × tabungan Anita + 30.000
100.000 = 2 × TA + 30.000
100.000 - 30.000 = 2TA
70.000 = 2 TA
TA = 70.000 / 2
TA = 35.000
jadi tabungan Anita = 35.000

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x : bilangan asli
karena 33 = jumlah bil. asli berurutan 
maka
x + (x+1) + (x+2) = 33
3x + 3 = 33
3x - 30 = 0

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p = (3x - 4)
le = (x + 1)

(a). K = 2p + 2le
      K = 2(3x - 4) + 2(x + 1)
      K = 6x - 8 + 2x + 2
      K = 8x - 6

(b). untuk x = 12, maka
K = 8(12) - 6
K = 90