✔ 最佳答案
x² - 6x - 16 = 0
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Factorisation
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When we have a function in the form of ax² + bx + c, to factorise we use the following steps:
Multiply a by c:
1 × -16 = -16
What we need to do is find what multiplies to make -16, but adds to give the middle term (b [-6] ). To do this we make a list of the factors until we find the right one:
1 × -16
2 × -8 <- This one adds to -6
We have two values: +2 and -8.
Method [ 1 ]:
Substitute these two values for our middle term (-6x), but as factors of x ( 2x and -8x ):
x² + 2x - 8x - 16 = 0
Imagine we have it split into two different parts:
x² + 2x | - 8x - 16 = 0
Factorise each part:
x( x + 2 ) -8( x + 2 ) = 0
*Note: if the two bracketed terms are not the same, you have done something wrong.
Now, we take one of the bracketed terms and the ubracketed terms:
( x + 2 )( x - 8 ) = 0
Method[ 2 ]:
Substitute this into the following unknown brackets:
( x _ ___ )( x _ ___ ) = 0
( x + 2 )( x - 8 ) = 0
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Now we can solve:
x + 2 = 0
x = -2
x - 8 = 0
x = 8
x = -2, 8
Note: Method[ 1 ] is necessary when term a is larger than 1 (i.e. 2x² or -2x² ). When term a is only 1 or -1, Method[ 2 ] will suffice.