tolong jawab bsk di kumpul dari no 6 - 9

7. E. 160
X - y = 12
X + y = 28
--------------- +
2X = 40
X = 40/2 = 20

X - y = 12
20 - y = 12
Y = 20 - 12 = 8

X x Y = 20 x 8 = 160:)

6. 
Misalkan
m = 1/x
n = 1/y
maka persamaan dapat ditulis
3m - 2n = 18
5m + 4n = 8

Eliminasi
3m - 2n = 18    |×2|    6m - 4n = 36
5m + 4n = 8     |×1|    5m + 4n = 8     +
                                --------------------------
                                      11m = 44
                                          m = 4
Subtitusi nilai m = 4 ke pers. 5m + 4n = 8
5m + 4n = 8
5(4) + 4n = 8
20 + 4n = 8
4n = -12
  n = -3

Jadi m = 1/x = 4
maka x = 1/4
n = 1/y = -3
maka y = -1/3

sehingga nilai x-y
x - y = 1/4 + 1/3
        = 3/12 + 4/12
       = 7/12  (B)

7.
x - y = 12     >>>   x = y + 12
x + y = 28

subtitusikan x = y + 12 ke pers. x + y = 28, menjadi
(y + 12) + y = 28
2y + 12 = 28
2y = 16
  y = 8

Subtitusikan nilai y = 8 ke pers. x - y = 12
x - y = 12
x - 8 = 12
x = 20

Jadi xy = (8)(20) = 160 (E)

8.
(x + 1) / (y + 4) = 1/2
(x - 1) / (y + 6) = 1/5
====================
(x + 1) / (y + 4) = 1/2
⇔ x + 1 = (y + 4) / 2
⇔ x = (y + 4 - 2) / 2
⇔ x = (y + 2) / 2 ...................(i)
=========================
(x - 1) / (y + 6) = 1/5
⇔ x - 1 = (y + 6) / 5
⇔ x = (y + 6 + 5) / 5
⇔ x = (y + 11) / 5 ......................(ii)
============================
maka (i) = (ii)
(y + 2) / 2 = (y + 11) / 5
5(y + 2) = 2(y + 11)
5y + 10 = 2y + 22
3y = 12
  y = 4

subtitusikan nilai y = 4 ke pers. x = (y + 2) / 2
x = (y + 2) / 2
x = (4 + 2) / 2
x = 6/2
x = 3

jadi pecahan yang dimaksud adalah 
x/y = 3/4
pecahan 3/4 dikali 8
= 6 (C)

9. 
2b + 3c = 300
b + 6c = 675

Eliminasi
2b + 2c = 300   |×3|    6b + 6c = 900
b + 6c = 675     |×1|      b + 6c = 675  -
                                --------------------------
                                        5b = 225
                                          b = 45
Subtitusikan nilai b = 45 ke pers. b + 6c = 675
b + 6c = 675
45 + 6c = 675
6c = 630
  c = 105

Jadi perbandingan harga 1 baju dan 1 celana adala
45 :105
= 3 : 7   (D)