x,y,z adalah barisan geometri dan x+y+z=7 jika xyz+27=0 maka nilai x-y-z=

barisan geometri
Un = ar^(n-1)

x = U1 = a
y = U2 = ar
z = U3 = ar²

xyz+27 = 0
a.ar.ar² + 27 = 0
a³r³ = -27
(ar)³ = (-3)³
ar = -3
a = -3/r

x+y+z = 7
a + ar + ar² = 7
a (1+r+r²) = 7
-3/r (1+r+r²) = 7
-3 -3r -3r² = 7
0 = 3r² + 3r + 10
(3r-1) (r+3)
didapatkan 
3r-1 = 0
3r = 1
r = 1/3
atau 
r+3 = 0
r = -3

gunakan terserah yg mana r nya harusnya hasilnya sama, saya gunakan yg r = -3

a = -3/r
a = -3/-3 = 1

maka
x = a = 1
y = ar = 1.(-3) = -3
z = ar² = 1.(-3)² = 9

jadi 
x-y-z = 1-(-3)-9 = -5