a) a+b=9 ,c=5
c·(a+b) ⇔ 5·9 = 45
b) a-b=4 ,c=10
c·(a-b) ⇔ 4·10 = 40
c) xz+yz=28 ,x+y=7
z·(x+y) = 28
z·7 = 28
z = 28:7
z = 4
d) xz-yz= 847 , z=11
z·(x-y) = 847
11 · (x-y) = 847
(x-y) = 847 : 11
x-y = 77
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