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jawab

12.
a = 1/x
b = 1/y

2/x + 1/y = 1  --> 2a + b = 1 ....(i)
1/x - 2/y = 8  --> a  - 2b = 8 ....(2)

(i dan ii)
2a + b = 1 ...kali 2
a - 2b = 8

4a + 2b = 2
a - 2b = 8 .....(+)
5a = 10
a = 2

b =  1- 2a
b = 1 - 4
b = - 3

a = 2 --> x = 1/2
b = - 3 --> y = - 1/3

nilai 3 / (x +y) = 3/ (1/2 - 1/3)  = 18

13.
n(B) = 5
banyak himpunan terdiri dari 4 anggota = 5C4 = 5

14e.
a= -3,
b = - 4
un = 107 --> a + (n-1) b = - 107
-3 + (n-1)(-4) = -107
(n-1)(-4) = - 104
n-1 = -104/-4 = 26
n = 27

14f.
a = 9
un = 99
sn = 540 --> n/2 (a + un) = 540
n/2 (9 + 99) = 540
n(54) = 540
n = 10

15.
biaya = 55x 2.000 = 110.000
Hasil penjualan = 32 x 3.000 + 23 x 3.500 = 176.500
Laba = 176.500 - 110.000 = 66.500 rp
%laba = 66.500/110.000 x 100% = 60,45%