use quadratic equation to solve r2 + 4r - 7 = 0

[alittleslower]

I do not understand how to use the quadratic equation to solve problems.

r2 + 4r - 7 = 0

I know I am supposed to substitute the numbers into the equation but I cannot come up with the same answer as my book. It just gives the answers and doesn't explain how to actually work the problem. Any help is greatly appreciated. Thank you very much!!

 

[skeeter]

it's time you learned the quadratic formula ...

for \(\displaystyle \L ax^2 + bx + c = 0\), \(\displaystyle \L x = \frac{-b \pm \sqrt{b^2-4ac}}{2a}\)

for your equation, r² + 4r - 7 = 0, a = 1, b = 4, and c = -7 ... plug and chug.

 

[jwpaine]

Grr.. teachers who have kids use the quadratic formula to solve for a non-factorable polynomial WITHOUT first showing them how to derive the quadratic formula... makes me mad.

Knowing how to complete the square before plugging and chugging is probably best.

http://www.purplemath.com/modules/sqrquad2.htm#formula

Tells you /how/ the formula is derived... and the first page tells you how to plug and chug.

 

[stapel]

I cannot come up with the same answer as my book.

We'll be glad to help you find any errors, but you'll need to show us what you did.

Thank you.

Eliz.

 

[Lost!]

My class just started the lesson on quadratic equations. I take notes and copy examples, but I am still very lost. I really need help starting a problem.

 

[stapel]

I really need help starting a problem.

With which bit do you need help?

For quadratic equations of the form "ax² + bx + c = 0", you've been given a formula, the Quadratic Formula, into which to plug "a", "b", and "c" to find the solution values of x. And you've been given links to lessons explaining the Quadratic Formula.

Are you not familiar with variables? Are you not able to tell which is the x²-term versus the linear (x) term versus the constant (strictly numerical) term, so you can't figure out which values are "a", "b", and "c"? Are you not familiar with working with formulas and evaluating for given values? Do you not know how to simplify radicals?

As you can see, there are many issues involved here. We'll be glad to find you links to lessons on whichever background topics you are needing help, but you really do need to specify (perhaps by showing your work...?) where you are having trouble.

Thank you for your consideration.

Eliz.

 

[Denis]

You've used 2 different names in this thread: alittleslower and lost!
Please don't do that: we can't tell who we're dealing with...

I'll give you an example:
2x^2 - 3x - 9 = 0 : solve for x

ax^2 + bx + c = 0
So in the example equation: a=2, b = -3, c = -9

Quadratic formula is: x = [-b +- sqrt(b^2 - 4ac)] / (2a)
Substitute above:
x = [-(-3) +- sqrt((-3)^2 - 4*2*(-9)] / (2*2)
x = [3 +- sqrt(9 + 72)] / 4

If you can't finish that, only your teacher can help you...