## justinee one year ago Can someone check this for me? (completing the square)

1. justinee

y = -2x^2 - 20x - 48

2. justinee

then i rewrote that into: y = -2(x^2 - 20x ____) -48 -20/2 = -10 then square it and I got 100 y = -2(x^2 - 20x +100) -48

3. justinee

then I added 100 to both sides and I got: y + 100= -2(x - 10)^2 -48 -100 -100 ------------------------ y = -2(x - 10)^2 - 148

4. halorazer

You subtracted 100 though ^

5. Decart

you add 48 to the other side and then also the 100 and the quadratic = 148

6. Decart

then you can factor the left side

7. justinee

can you show me that in steps? like from the beginning

8. Decart

you did it rite just set the equation = 0 then subtract 49 from both sides so 2x^2-20x=48

9. Decart

then complete the square and add the 100 to both sides

10. justinee

so it'd be 0 = -2(x^2 - 20x + 100) -48 ?

11. justinee

why do I need to factor that

12. justinee

Ok I got it thanks! and one more question

13. justinee

How would I solve: x^2 - 38x + c also by completing the square

14. Decart

c is a constant most likely just move it to the other side and complete the square

15. justinee

what is that going to give me?

16. Decart

x^2-38x=c

17. justinee

and what would be the next step

18. justinee

0 = (x2 - 38x ___) +c ?

19. Decart

square the -38 and do the same to both sides

20. Decart

(x^2-38x+__)=__-c

21. justinee

-38/2 = -19^2 = 361

22. Decart

yup

23. Decart

so that is on both sides

24. Decart

(x^2-38x+361)=361-c

25. justinee

okay so far i have: (x^2 - 38x + 361) = 361 -c

26. Decart

right now you should be able to factor the quadratic which is the point of completing the square

27. justinee

so like: ____ x___ = 361 ____+___= -38

28. Decart

right it will be the same number it is squared

29. justinee

-19

30. Decart

yup

31. justinee

(x-19)^2 = 361 - c?

32. Decart

and the first equation was x^2-10+25=49

33. Decart

right

34. justinee

what do I do now?

35. Decart

with the second you can't do anything it is most likely to show how you did the work

36. Decart

x-5=7

37. justinee

so just that ^

38. justinee

how would i write tht in intercept form

39. Decart

do you mean y=mx+b

40. justinee

no I also have to rewrite tht in intercept

41. Decart

the x intercept is +or-(7/5,0)

42. Decart

that is the y is 0