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dan biaya per/siswa mula-mula = Rp y
maka xy = .3000.000 .... (1)
setelah bergabung 10 orang, maka :
(x+10)(y - 15.000) = 3.000.000
xy - 15000x + 10y - 150.000 = 3000.000
karena xy = 3000.000, berarti
3000.000 - 15.000x + 10y - 150.000 = 3.000.000
10y = 15.000x + 150.000
y = 1500x + 15.000 ... (2)
substitusi, (2) ke pers (1), get
x(1500x + 15.000) = 3.000.000
1500x² + 15.000x - 3.000.000 = 0
x² + 10x - 2000 = 0
(x - 40)(x + 50) = 0
pembuat nol yg positif :
x - 40 = 0
x = 40
substitusi x = 40 ke pers (1), get
xy = 3000.000
y = 3000.000/x = 3.000.000/40
y = 75.000
jadi, bnyak siswa sekarang = 40+10 = 50 orang
dan biaya per/siswa sekarang = 75.000 - 15.000 = Rp 60.000