Question

Please help. Which is correct.
Solve -2x2 +3x - 9 = 0

x equals quantity of 3 plus or minus 3i square root of 7 all over 4

x equals quantity of 3 plus or minus 9i square root of 7 all over 4

x equals quantity of negative 3 plus or minus 3i square root of 7 all over 4

x equals quantity of negative 3 plus or minus 9i square root of 7 all over 4

I will give medal :)

Answer 1

@insa help?

Answer 2

umm....i have no idea.. havent studied it yet :/

Answer 3

Aww haha ok, thanks anyways @insa

Answer 4

haha :D

Answer 5

Please actually solve the equation -2x2 +3x - 9 = 0.

You could make it easier for yourself by changing all of the signs:

2x2 - 3x + 9 = 0

How would you go about solving this?

Hints: Some appropriate methods are
1. factoring
2. quadratic formula
3. graphing

Answer 6

well the thing is, I have not really learned this yet? its a pre-study guide so could you help me? @mathmale

Answer 7

Are you familiar with the quadratic formula?

If so, type it out here.

Answer 8

isn't it something like x= -b + sqrt b^2 -4ac/2a?

Answer 9

@mathmale

Answer 10

That's right! Good.

Obtain your values of a, b and c from your original posting: -2x2 +3x - 9

Answer 11

ummm

x=-3 + sqrt 3^2 -4(-2)9/2a ?? lol Im sorry if its wrong 0.0 @mathmale

Answer 12

Just compare:
-2x2 +3x - 9 to
ax^2 +bx + c

and decide upon the values of a, b and c.

Answer 13

Hint: c = -9. a = ? b = ??

Answer 14

a= 2 and b=3? @mathmale

Answer 15

*-2

Answer 16

Yes:
a=-2
b=3
c=-9

Now the quantity under the radical sign is called the "determinant," and the "determinant" equals

b^2 - 4*a*c.

Bet you've seen this before.

Substitute the values of a, b and c in to this formula and tell me what you get.

Answer 17

b^2, when b=3, is:

(3)^2 = ?

Answer 18

that would equal 9?

Answer 19

Yes, and -4 ( a )*( c ) = ?

Answer 20

a= 2 and c= -9
so
-4(2)*(-9)
-8*(-9)
=-72? ._.

Answer 21

Yes, and so

your b^2 - 4ac has what value? Go back and take what you've already found for b^2 and -4ac.

Answer 22

9-72? is that what you mean

Answer 23

Indeed it is. 9-72= ?

Answer 24

thats -63

Answer 25

that's right. Can we find the square root of a negative number?

Answer 26

7.93725393 i

Answer 27

But that 63 is negative. Can we find the square root of a negative number?

Answer 28

I'm guessing no?

Answer 29

Are you familiar with the operator " i " found in situations such as this one?

If you have Sqrt(-63), you re-write it as i*Sqrt(63). Have you seen that before?

Answer 30

i believe so

Answer 31

" i " is called the "imaginary operator."

Answer 32

oh ok

Answer 33

So, our Sqrt(-63) comes out to\[i*3*\sqrt{7}\]
because sqrt(9)=3. Make sense to you?

Answer 34

yes :)

Answer 35

Great!

Look at the four answer choices:

x equals quantity of 3 plus or minus 3i square root of 7 all over 4

x equals quantity of 3 plus or minus 9i square root of 7 all over 4

x equals quantity of negative 3 plus or minus 3i square root of 7 all over 4

x equals quantity of negative 3 plus or minus 9i square root of 7 all over 4


We have to decide which one is correct.

Note that your quadratic formula comes out to \[x=\frac{ -b. plus.or.minus.Sqrt(4ac) }{ 2a }\]and that you already know that Sqrt(4ac) = i*3*Sqrt(7).

Answer 36

You know the values of b and c.

So, would you write out your own expression for the roots, x, and then compare y our result to the four possible answers?

Answer 37

i would say it would be the 2nd one?

Answer 38

Have you actually written out your solution statement,

x=(-b plus or minus ) Sqrt(b^2 - 4ac) ) / 2a ?

Answer 39

wait, so it would be the first one then right?!

Answer 40

Hold it, please...I know you want to finish as soon as possible, but I'd really appreciate your answering my question: have you actually written out your solution statement?

Answer 41

yes

Answer 42

and you think that the first of the four possible answers matches your result?

Answer 43

yes i do

Answer 44

but isn't b = 3, so that -b = -(3) = -3 ? Wouldn't that eliminate the first 2 answer choices?

Answer 45

OH! i completely forgot the - I'm sorry

Answer 46

In the end I'm going to let you make the choice of which answer is best. You've done very well here.

Answer 47

I got it now but thank you so much! @mathmale :))))))

Answer 48

We went through a lot of effort, but that's really the only way to learn this material.

You're very welcome!!! Hope to see you again soon on OpenStudy. :)