P1( 3 | 7)
P2 ( -2 | 4 )
P3( 1 | 5 )
f ( x ) = a * x^2 + b*x + x
f ( 3 ) = a *3^2 + b * 3 + c = 7
f ( 3 ) = 9a + 3b + c = 7
f ( -2 ) = a *(-2)^2 +b*(-2) + c = 4
f ( -2 ) = 4a - 2b + c = 4
f ( 1 ) = a *1^2 + b * 1 + c = 5
f ( 1 ) = a + b + c = 5
9a + 3b + c = 7
4a - 2b + c = 4
a + b + c = 5
1 minus 2 :
5a + 5b = 3
2 minus 3 :
3a - 3b = 5
5a + 5b = 3 | * 3
3a - 3b = 5 | * 5
15a + 15b = 9
15a - 15b = 25 | abziehen
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30b = -16
b = -16/30
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