Question

Solve 3x^2 + 21x = -30

Answer 1

do you know the quadratic formula?

Answer 2

no

Answer 3

ok let me write it out

Answer 4

actually here

this is the formula take a look at it

http://2.bp.blogspot.com/_V8KsSIiGjBk/SsACMEj73KI/AAAAAAAAFIU/vNtErLdchMw/s1600/Quadratic+Formula.gif

Answer 5

now in order to use this formula you need to make the equation you have

= 0

do you know how to do that?

Answer 6

its \[x=(-21\pm \sqrt{21^{2}-(4(3)(30))} )/2(3)\]

Answer 7

if you don know how to make your equation = 0

i can help you

Answer 8

these are the answer options
x = 6 and x = 5

x = –5 and x = –2

x = 7 and x = –5

x = –6 and x = –2

Answer 9

ok but first you have to make the equation

3x^2 + 21x = -30

= 0 instead of = -30

do you get how to do that?

Answer 10

no

Answer 11

could you explain please?

Answer 12

ok so the equation

3x^2 + 21x = -30

at the moment = -30

to get rid of -30 and make it = 0

we add 30 to the other side

which will make the equation

3x^2 + 21x + 30 = 0

Answer 13

alright.. then what do i do next?

Answer 14

ok so now to use the formula we need to know the values of a, b and c to input that in the formula

Answer 15

k...

Answer 16

and remember the equation should be in this order:

ax^2 + bx + c = 0

Answer 17

okay

Answer 18

which it already is

Answer 19

the equation is

3x^2 + 21x + 30 = 0

and the value of A is the first number we have there which is 3
the value of B is the second number we have there which is 21
the value of C is the third number we have there which is 30

remember if any of the numbers were - it would be a minus value
e.g. if the third number was -30, then C would be -30

so now we have the values

a = 3
b = 21
c = 30

Answer 20

now we need to place this in the formula do you understand how to do that if not tell me i will go through the steps with you

Answer 21

no i dont know how to find that

Answer 22

ok have you taken a look at the formula i showed you on

http://2.bp.blogspot.com/_V8KsSIiGjBk/SsACMEj73KI/AAAAAAAAFIU/vNtErLdchMw/s1600/Quadratic+Formula.gif

Answer 23

okay got it

Answer 24

ok so first thing we need to do is but the value for b where it says -b

so we know the value for b is 21

but because it says -b we have to change 21 to -21
(but remember if the value of b was -21 and we had to put it where -b is it would change to a positive)

so do you understand that so far that where it says -b we put -21

Answer 25

yes i understand

Answer 26

ok so now where the square root sign is

for b^2 we put in 21^2 or 441

Answer 27

and for 4ac we put in

4 * a * c

which is

4 * 3 * 30

Answer 28

so now we have the values in the square root sign we can work out what should be there

by doing

b^2 - 4ac

which is

441 - 360

which would be

\[\sqrt{81}\]

Answer 29

do you understand everything so far?

Answer 30

yes

Answer 31

ok so square root of 81 is 9

so we got rid of the square root sign and we have just 9 there

so our formula is

now

\[x -21 \pm 9 \over 2a\]

Answer 32

the value for 2a is simply

2 * a

which is 2 * 3

which is 6

Answer 33

so our formula now looks like this

\[x = {-21 \pm 9 \over 6}\]

Answer 34

now you would have noticed there is a + and - sign in between -21 and 9 which means we have to do both things and this will give us two different answers

Answer 35

do you know how to work this out now?

Answer 36

yes thanks

Answer 37

dont forget to click good answer :) and your welcome