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# Completing the square watch

1. Hi,
can someone tell me where I have gone wrong because I get a square root of negative 15, which is impossible.

x2+8x-1=0
x2+8x=1
(x+4)2=-15
x+4=square root of -15
2. (Original post by Louise_1988)
Hi,
can someone tell me where I have gone wrong because I get a square root of negative 15, which is impossible.

x2+8x-1=0
x2+8x=1
(x+4)2=-15
x+4=square root of -15

x^2+8x-1=0
(x+4)^2-17=0
x+4 = +/- square root of 17
x = - 4 +/- square root of 17.
3. (Original post by Louise_1988)
Hi,
can someone tell me where I have gone wrong because I get a square root of negative 15, which is impossible.

x2+8x-1=0
x2+8x=1
(x+4)2=-15
x+4=square root of -15
x^2 + 8x - 1 = 0

Completing the square: (x + 4)^2

Multiplying this out: x^2 + 8x + 16 - you need to take away 17 to get -1

=> (x + 4)^2 - 17 = 0
4. (Original post by Louise_1988)
Hi,
can someone tell me where I have gone wrong because I get a square root of negative 15, which is impossible.

x2+8x-1=0
x2+8x=1
(x+4)2=-15
x+4=square root of -15
you take 4^2 from the same side, so it becomes -16 - 1 = -17 then move it across.
5. (Original post by Louise_1988)
Hi,
can someone tell me where I have gone wrong because I get a square root of negative 15, which is impossible.

x2+8x-1=0
x2+8x=1
(x+4)2=-15
x+4=square root of -15
technically root(-15) isnt impossible but thats a whole other story

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