SRL Archive - Mathematics

7x^2+20x-40=0

Show your work :p

What do you want done with it?… I will assume that you mean solve for x… correct?

7x^2+20x-40=0 {using Quadratic Formula}

(-20(+/-)Sqrt[4400-(4*7*(-40))])/(2*7)

(-20(+/-)Sqrt[4400-(-1120)])/(14)

(-20(+/-)Sqrt[5520])/(14)

(-20(+/-)4Sqrt[345])/(14)

(-10(+/-)2Sqrt[345])/(2)

x= (-10+2Sqrt[345])/(2) and/or (-10-2Sqrt[345])/(2)

Or, if you have a TI 83 or up, you can use the solver.

MATH -> 0 -> CLEAR -> Enter the Equation -> ENTER -> 100 -> ALPHA -> ENTER

Will give you the answer closest to 100, which is the positive number. Then repeat for -100, with will give you the lower answer, because quadratics have two answers.

=+1.356
=-4.213

Thats what I'd do on an exam/test to save time.

and then you'll get 1/4 credit for no work, if your teacher is even nice enough to give you 1/4…

My teacher: No work = No Credit. :(

From: Brain
and then you'll get 1/4 credit for no work, if your teacher is even nice enough to give you 1/4…

Not me. The teacher doesn't mark on how you did it. You get it right = full points.

The only time that it comes down to work even mattering, is when you get the answer wrong, and he can give you work for doing some of the steps right.

IB marks really weird though, and the questions are so hard that they pretty much encourage shortcuts. If you can do it, and it saves time, do it.

offtopic: my teacher just wants 3/4 of the thing done, and you get full credit. even if its wrong with no work lol

also, we just "learned"(more like reveiw) this stuff in my math class, lol

I am in a math class that does not allow the use of calculators… which makes geometry a bit tough… graphing.. and SquareRoots..

I've gone from if your notes aren't done with a ruler you fail" to "as along as you tried"

Comparing to you all, I have it easy. My teacher gives partial credit for work, but wrong answer, but it's totally wrong if you have no work. I don't ever need to show my work (maybe it's just me, highest average in class ftw). We can use always use calculators.

Also, do your own homework.

x~=-4.21337
or
x~=1.35623

Here's my work :P

From: 3Garrett3
Or, if you have a TI 83 or up, you can use the solver.

MATH -> 0 -> CLEAR -> Enter the Equation -> ENTER -> 100 -> ALPHA -> ENTER

Will give you the answer closest to 100, which is the positive number. Then repeat for -100, with will give you the lower answer, because quadratics have two answers.

=+1.356
=-4.213

Thats what I'd do on an exam/test to save time.

Thanks for that. It saved me a bunch of time on a test I took Today.

From: Krypton48
7x^2+20x-40=0

Show your work :p

Anything you will ever need :)

http://en.wikipedia.org/wiki/Quadratic_equation

Without using the quadratic formula (completing the squares).

7x^2+20x-40=0
7x^2+20x=40
x^2 + 20x/7 = 40/7
x^2 + 20x/7 + 100/49 = 40/7 + 100/49
(x + 10/7)^2 = 380/49
x + 10/7 = +-sqrt(380/49)
x = +-sqrt(380)/7 - 10/7
x = (-10 + sqrt(380))/7 or (-10 - sqrt(380))/7

Tada, same answer as everyone else.