To prove that a/(x-1)+b/(x-3) = 0 has a solution in the
interval (1 ,3).
Multiply both sides of the give equation
by (x-1)(x-3):
a(x-3) + b(x-1) =
0.
ax-3a+bx-b = 0.
Add 3a+b to
both sides:
ax+bx =
3a+b.
x(a+b) = 3a+b.
x =
(3a+b)/(a+b).
x =
{3(a/b)+1}/(a/b+1)........(1)
Since ab> 0, a and b
are of the same sign. Therefore 0 < a/b < 1 Or a/b
>1.
If a/b> 1, then 2
<{3a/b+1}/(a/b +1) <
3....................(2)
If a/b = 1, then (3a/b +1)/(a/b
+1) = (3+1)/(1+1) = 2.....(3)
If 0 < a/b <1 ,
then 1 < {3(a/b) +1}/((a/b)+1) <
2..........(4)
From (1) and (2) we see that whenab
> 0 and a>b there is solution in (2,3)
.
From (1) and (3) when ab> 0 a = b , there is a
solution x =2.
From (1) and (4) , when ab> 0, a
< b rhere is a solution fox in (1 , 2).
Combining
the 3 cases, when ab> 0 , x has a solution in (1,3)
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