completing the square

[mollykins]

I can do some of them, but on mymaths, there are questions that involve ending up with fractions, but we haven't ben taught how to do these, and the online tutorial doesn't include any that do . . . . can anyone help?

[factor-fiction]

The mymaths page I'm looking at has some worked examples. it takes you through them step by step.
Here's a link, although I think you'll need your mymaths password?
http://www.mymaths.co...quationsWithFractions

[Prudie]

That example looks like solve with the formula x= -b+ or - Sqr root (b squared - 4ac) / 2a

[factor-fiction]

16x²-72x-9=0
16x²-72x +81 -81 -9=0
(4x- 9 )² -90 =0
(4x-9) = √90
(4x-9) = 3√10
4x = 3√10 +9
Divide each term by 4 to find x
x = (3/4)√10 + 9/4


The method is right but in my haste I may have mistyped something somewhere or slipped up

[morn]

once the brightest sparks in our class were pulled out for an exam for a particular prize. the next day the head of department appeared - full of apologies: he thought we'd been taught what they'd been asked, and only then realised we hadn't. You need to tell your teachers you haven't been taught fractions

[mollykins]

Thanks factor, at least you know it's highly unlikely i'll be able to copy that answer into one of the questions, but at least i can see how it's done now :D

She knows we haven't been taught fractions, perhaps she didnt realsie there were one's with them in this exercise.

[vascop]

Here's my contribution. I'm trying to make it clearer how you solve these in general. Don't forget that every square root has 2 values +/-. That's how you get the 2 answers for x.

16x²-72x-9=0 (ax^2+bx+c=0) a=16, b=-72 and c=-9
16x²-72x +81 -81 -9=0
(4x- 9 )² -90 =0
The 81 in the first line is calculated as
The 4 in the bracket calculated as sqrt(a)=sqrt(16)=4
The -9 inside the bracket is calculated like this:
b/(2 x a)=-72/(2 x 4)=-9
The 81 in the first line is calculated as this number squared (-9)^2=81
The -9 in the original equation has nothing to do with this one.

The line below should be plus or minus sqrt(90) to get 2 answers for x:
(4x-9) = +√90 or (4x-9) = -√90
(4x-9) = +/-3√10
4x = +/-3√10 +9
Divide each term by 4 to find x
x =+/- (3/4)√10 + 9/4

[mollykins]

Thanks . . . . xx molly :D