How do you solve 3x^2+5x-2=0?

3 Answers
Aug 15, 2016

x = -2 or x = 1/3

Explanation:

Factorise to get (3x - 1)(x + 2) = 0

hence
3x - 1 = 0 or x + 2 = 0

giving
x = 1/3 or x = -2

Aug 15, 2016

x = 1/3 or x = -2

Explanation:

Find factors of 3 and 2 which subtract to make 5.

We should realise that (3x2) - 1 = 5.
Find the cross products and subtract to get 5.

3" "1 " "rArr 1xx1 =1
1" "2" "rArr 3xx2 = 6 " "(6-1 = 5)

Add in the signs.

3" "-1 " "rArr 1xx-1 =-1
1" "+2" "rArr 3xx+2 = +6 " "(+6-1 = +5)

(3x-1)(x+2) = 0
One of the factors must be 0.

If 3x-1 = 0, " then " x = 1/3
If x+2 = 0, " then " x = -2

Aug 15, 2016

There is only one way to deal with factorization. That is lots of practice.
x=+1/3

x=-2

Explanation:

Given:" "3x^2+5x-2=0
'............................................................................................................

Both 3 and 2 are prime numbers so they can only be the products of:

for 2->1xx2
for 3->1xx3

So without taking any notice of the signs (sorted out afterwards) we have something like: color(red)((3x+1)(x+2) " or "(3x+2)(x+1))
I have kept both 3x" and "x positive.

color(red)("Note that the signs in the above will be wrong!")

color(green)("Ok. Lets consider the signs by looking at the "+5x) color(green)("from "3x^2+5x-2)

Notice that
(+3x)xx(+2)=6x
(-1)xx x=-x

giving 6x-x=+5x as required

So the factor form is very likely:

(3x-1)(x+2)

'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Check:
Multiplying out the brackets
color(blue)((3x-1))color(brown)((x+2) )" "rarr" " color(brown)(color(blue)(3x)(x+2)color(blue)(" "-1)(x+2))

" "=3x^2+6x" "-x-2

=3x^2+5x-2 as required
'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
So we have:

3x^2+5x-2=0" "->" "(3x-1)(x+2)=0

Note that anything xx0=0

This means that the solution is such that

(3x-1)=0 -> x=+1/3

(x+2)=0->x=-2

Tony BTony B