Let f(x)= y 0 (1- (xy)) (xy)y3. Then the number of solutions of the equation f(x)= x, x R is :
- 0
- 2
- 3
- 1
Answer
(3) 3
Solution
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For x 0: f(x)= y 0 [ 1- (xy)(xy)2 (xy)xy x3 ]; for x=0, f(0)=0
0 1- 2= 12 and 0 =1
f(x)= x32 for all x
Let h(x)= x32- x; h is odd and h(0)=0.
h(1)=0.5- 1<0, h ( 2 )= 316-1>0 a root in (1, 2 )
h'(x)= 3x22- x increases on (0, 2 ) from -1, so h first decreases then increases; for x 2, x32>1 x
exactly one positive root, and (h odd) exactly one negative root
Number of solutions =1+1+1=3
Answer: (3)