## anonymous one year ago what is the solution of the matrix ?

1. anonymous

2. xapproachesinfinity

do you have to do it with matrices?

3. anonymous

um i think soo , would you do this the same way you would do it if it was addtion ?if that makes sense

4. anonymous

5. anonymous

ill show you what im talking about

6. anonymous

this is what i am talking about would i do it the same but just multiply ?

7. xapproachesinfinity

not really getting your point inverse of 2x2 matrix can be computed like this |dw:1433291928663:dw|

8. xapproachesinfinity

i see your point, you need inverse and them multiplciation operation of matrices there is not addition here

9. perl

$$\Large {A}\mathbf{ x} = \mathbf{b} \\ \Large \mathbf{ x} = A^{-1} \mathbf{b}$$

10. anonymous

I know ...... :(((((

11. xapproachesinfinity

try to find the determinant first

12. anonymous

now how would i find that , i usually use a online matrix calculator to find the determinant so i wouldn't know how to find on my own

13. xapproachesinfinity

ok the determinant is computed like this |dw:1433292400098:dw|

14. anonymous

ok

15. xapproachesinfinity

you multiply ad and bc and then subtract

16. anonymous

ok thats all ?

17. xapproachesinfinity

do the same with your matrix

18. xapproachesinfinity

no not finished that's only how to do determinant

19. anonymous

ok im writing this down

20. xapproachesinfinity

ok

21. anonymous

cross multiply ?

22. xapproachesinfinity

yeah seems like cross multiply then take the difference

23. anonymous

so for me it would me 9*1 &4*2 for A and for X it would be -9*-8 and -6 *-1 right?

24. anonymous

or did you mean multiply A with X ???

25. xapproachesinfinity

so here $A=\begin{bmatrix}9&4\\ 2&1\end{bmatrix}$ $\det(A)=9(1)-2(4)=9-8=1$

26. xapproachesinfinity

oh no what are you taling about? are you following what i'm saying haha

27. anonymous

yeah

28. anonymous

9*1 i did that 2*4 i did that too

29. xapproachesinfinity

we are looking for X and i used what perl did X=Ainverse times B

30. anonymous

i know what i did wrong i forgot to subtract ..

31. xapproachesinfinity

it was AX=B then we multiplied A^-1 and we got X=A^{-1}B now we are in the process of computing A inverse that is why we need determinant of A

32. xapproachesinfinity

no seems you missed the whole poiint from what i understand from your last reply

33. anonymous

no .. i didn't you said cross multiply then subtract right ???

34. xapproachesinfinity

that's only for determinant

35. xapproachesinfinity

i did that for you already see my replay above

36. xapproachesinfinity

$$\color{blue}{\text{Originally Posted by}}$$ @xapproachesinfinity so here $A=\begin{bmatrix}9&4\\ 2&1\end{bmatrix}$ $\det(A)=9(1)-2(4)=9-8=1$ $$\color{blue}{\text{End of Quote}}$$

37. anonymous

ok so what do have to do next then ?

38. anonymous

find x ?

39. xapproachesinfinity

|dw:1433293368759:dw|

40. anonymous

yup

41. xapproachesinfinity

so we do 1/det A so 1/1 =1 here we didn't need to worrt about it since it is just 1

42. anonymous

ok

43. xapproachesinfinity

next step is to do a switch|dw:1433293517899:dw|

44. xapproachesinfinity

you take A and switch a and d b and no switch you just change the sign

45. anonymous

ok i get that but where do the negatives come from

46. xapproachesinfinity

47. xapproachesinfinity

what kind of class are taking

48. anonymous

algebra 2

49. xapproachesinfinity

hmm really? i don't really know how much you know about matrices

50. xapproachesinfinity

hmm this really needs some time to get accustomed to! it may be fairly easy, but needs a lot of practice you might want to look at some video tutorial on youtube

51. anonymous

okay , i will

52. xapproachesinfinity

once you feel comfortable then tackle problem it is no helping to do problem you are not ready to do be real, no matter how much i explain to you here, you will need the lecture first here open study will not give you the lecture rather help assist you with difficulties you are having after knowing the lecture and know fairly enough on the subject

53. xapproachesinfinity

i will do your problem now, but next problem you need to try your hardest :)

54. anonymous

okay

55. xapproachesinfinity

let me do it on paper ans take a picture

56. anonymous

ok

57. xapproachesinfinity

almost hehe

58. anonymous

kay

59. xapproachesinfinity

60. xapproachesinfinity

before the last line i preformed matrix multiplication

61. xapproachesinfinity

62. anonymous

yeah it is . -5 ,26 -9 and -60 correct right

63. xapproachesinfinity

there is no -9

64. anonymous

oh ,

65. xapproachesinfinity

that's just brackets

66. xapproachesinfinity

that should match C

67. anonymous

oh , my bad it looked like a negative for a second but yes it is C

68. xapproachesinfinity

okay

69. xapproachesinfinity

hope that helps

70. anonymous

it does ,,,,, THANKS ..

71. xapproachesinfinity

my pleasure