(x y) D . 6 X .3 A B .ABC f(x) a b x y z yz + xz + xy = 1: X Y

1991 ' "
10
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5
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.1
:
5
,
3
.
.
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(x; y)

.2
x(5y ; 7) = y + 2:
2

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ABC
f (x)
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; f (1) = 3 (i)
a; b
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.
.
X .3
d X
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f (a + b) + f (a ; b) = 2[f (a) + f (b)]:
f (x)
.

3(x + 1) = 4(y + 1) = 5(z + 1)
x
y
z
yz + xz + xy = 1:
2
:
2
Z = FY (X )
90

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2

X; Y
Y X

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-
(i)
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1.9
Y Z = XY
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-
.
P = FA (P ); P = FB (P ); P = FC (P ); P = FD (P ):
1
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2
1
3
2
4
3
: .
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.
P
4
-
P
0

P
26
0
BD
P P
4 -
-
0

AC (i)
(ii)

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