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substitusikan x = 1, get
1³ - a(1)² + b(1) + 3a = 0
1- a + b + 3a = 0
2a + b = -1 ... (1)
substitusikan x = 3, get
3³ - a(3)² + b(3) + 3a = 0
27 - 9a + 3b + 3a = 0
-6a + 3b = -27
2a - b = 9 ... (2)
eliminasi pers (1) dan 2 :
2a + b = -1
2a - b = 9
--------------- (+)
4a = 8
a = 2
substitusikan a = 2,
2a + b = -1
2(2) + b = -1
4 + b = -1
b = -5
shg, persamaan di atas dpt menjadi :
x³ - 2x² - 5x + 6 = 0
x1 + x2 + x3 = -b/a = -(-2)/1 = 2
1 + 3 + x3 = 2
4 + x3 = 2
x3 = 2 - 4
x3 = -2
jadi, akar yg letiga adalah = - 2