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Quadratic formula gone wrong:( help!

I'm practising quad.formula questions and i really cant figure out what i'm doing wrong. I know i'm doing wrong because i know the answer and i cant get there :frown:

This is my question and answer:
Find x from this equation using the quadratic formula:
x^2+10x-4=0
My answer:
x= -10±√10^2+(4x1x-4)
---------------------
2x1
+√=-0.42 -√=-9.58

ACTUAL answer:
+√=0.38 -√-10.39

Please help me it driving me crazy!

Reply 1

ch0wm4n

Original post by Prada16

I'm practising quad.formula questions and i really cant figure out what i'm doing wrong. I know i'm doing wrong because i know the answer and i cant get there :frown:

You've made a sign error i think

\dfrac {-b +/- \sqrt(b^2-4ac)}{2a}

Edit: sorry i couldn't get the ± sign in :s-smilie:

(edited 11 years ago)

Reply 2

cambo211

X= (-b ± √(b^2 - 4ac))/2a

Just a sign error.

(edited 11 years ago)

Reply 3

Prada16

nope i havent if A or C is a minus then -4ac turns into +4ac i've tried both +4ac just makes my answer further away than the actual answer :frown:

Reply 4

Happy-Turtle

this is how i worked it out

x^2+10x-4=0

quadratic formula:
-b + or - √b^2 - 4ac / 2a

so
a= 1, b= 10, c=-4

-10 + or - √100 - (4x1x-4) / 2

-10 + √116 / 2 = 0.38

-10 - √116 / 2 = -10.39

Reply 5

The Nosy Gecko

pretty sure you made a sign error, always make sure you write the - sign out in the quad formula before your answer to 4ac as sometimes you miss one out.

e.g.

the 4ac here = -16

so inside the square root it will be 100-(-16) not 100-16

Reply 6

Prada16

Arg got it now stupid revision guide has got it wrong way round say to MAKE it +4ac when u have a -sign in a or c not that it becomes + coz of the,-&- grrrrr stupid thing :frown:

thanks everyone :biggrin: