## eli123 3 years ago factor 21x^2-49x-9x-21

1. eli123

idk how i got 7x(3x-7) 3(3x-7) and i know its wrong

2. hartnn

is the original problem 21x^2-58x-21 ?

3. eli123

no

4. eli123

5. hartnn

right,so thats 21x^2-40x-21=0 right ?

6. eli123

yeah and then the product of 21 and -21 is -441 and then 40 so the two numbers are -49 and 9

7. hartnn

that is correct, see above u have written -49x - 9x instead of what u got now as -49x+9x

8. eli123

oooh i see wow

9. hartnn

now whats common from first 2 terms in 21x^2-49x-9x-21 =0?

10. eli123

7x

11. eli123

i got the answer :P -3/7 and 7/3

12. hartnn

yup, correct.

13. eli123

im having problems with another equation

14. hartnn

which ?

15. eli123

16. eli123

i got to the point 9 + or - sqrt of 1 all over 4

17. hartnn

9 from where ?

18. hartnn

Compare your quadratic equation with $$ax^2+bx+c=0$$ find a,b,c then the two rots of x are: $$\huge{x=\frac{-b \pm \sqrt{b^2-4ac}}{2a}}$$

19. eli123

yeah b is 9

20. hartnn

nopes, b is -4

21. eli123

oh wait nooo i gave you the wrong equation! no wonder

22. hartnn

lol :P

23. eli123

24. hartnn

still b is -9

25. eli123

but in that equation it says -b so -(-9) is 9

26. eli123

inside i did put put -9^2 = 81

27. hartnn

yes, u are correct $$\huge\frac{9 \pm \sqrt{1}}{4}=\frac{10}{4},\frac{8}{4}$$

28. eli123

wait you are just adding and subtracting the 1?

29. eli123

obviously lol never mind i feel stupid

30. hartnn

got it now ?

31. eli123

yes, it was a stupid moment :P

32. eli123

ok i did another one and i got 4+-sqrt of 28 all over 2 @hartnn

33. hartnn

question ?

34. eli123

its the first one that sent you

35. eli123

36. hartnn

thats correct :)

37. eli123

but how do i factor it?

38. hartnn

do u want to factor or do u want to use quadratic formula? anyways its not factorable....

39. eli123

so what would be the answer?

40. eli123

i cant simplify it?

41. hartnn

all u can do is $$2 \pm \sqrt7$$

42. hartnn

$$\sqrt{28}=2\sqrt7$$

43. eli123

how does that equal to that

44. hartnn

$$\sqrt{28}=\sqrt{7*4}=\sqrt{7}\sqrt{4}=2\sqrt{7}$$

45. eli123

got it

46. eli123

you are the best!

47. hartnn

lol. glad to hear that :) u too are awesome ;)

48. eli123

lol thank you, well i gotta keep doing this stupid hw

49. hartnn

best of luck for that.

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