Question Details

Let the function  f : [ 1 , ) R be defined by

f ( t ) = { ( 1 ) n + 1 2 ,  if  t = 2 n 1 , n N ( 2 n + 1 t ) 2 f ( 2 n 1 ) + ( t ( 2 n 1 ) ) 2 f ( 2 n + 1 ) ,  if  2 n 1 < t < 2 n + 1 , n N

Define  g ( x ) = 1 x f ( t ) d t , x ( 1 , ) . Let α denote the number of solutions of the equation g(x) = 0 in the interval  ( 1 , 8 ] and  β = lim x 1 + g ( x ) x 1 . Then the value of α + β is equal to ______.

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Correct Answer :

5

Solution :

First, let us analyze the definition of the function f:[1,)R.

For odd integers of the form t=2n1 where nN, we have:
f(2n1)=(1)n+12

Evaluating this at consecutive values of n gives:
- For n=1: t=1f(1)=(1)22=2
- For n=2: t=3f(3)=(1)32=2
- For n=3: t=5f(5)=(1)42=2
- For n=4: t=7f(7)=(���1)52=2

For open intervals 2n1<t<2n+1, the function is defined as:
f(t)=2n+1t2f(2n1)+t(2n1)2f(2n+1)

This expression represents the linear interpolation between the points (2n1,f(2n1)) and (2n+1,f(2n+1)). Thus, the function f(t) is a continuous, piecewise linear (sawtooth-like) function connecting the vertices:
(1,2),(3,2),(5,2),(7,2),(9,2),

Let us determine the value of β:
β=limx1+g(x)x1

Since g(1)=11f(t)dt=0, by the definition of the derivative and the Fundamental Theorem of Calculus:
β=g(1+)=f(1)=2

Next, let us analyze the equation g(x)=0 for x(1,8].

1. For the interval t[1,3]:
The line segment connecting (1,2) and (3,2) is given by:
f(t)=22(t1)=42t
Integrating to find g(x) on this interval:
g(x)=1x(42t)dt=[4tt2]1x=x2+4x3
Setting g(x)=0(x1)(x3)=0.
Since x(1,8], this yields one solution: x=3.
Note that g(3)=0.

2. For the interval t[3,5]:
The line segment connecting (3,2) and (5,2) is:
f(t)=2+2(t3)=2t8
Integrating for x[3,5]:
g(x)=g(3)+3x(2t8)dt=0+[t28)]3x=x28x+15
Setting g(x)=0(x3)(x5)=0.
Since x>3 in this sub-interval, this yields the solution: x=5.
Note that g(5)=0.

3. For the interval t[5,7]:
Due to symmetry and periodicity of the sawtooth function (shifted by 4 units), the behavior of g(x) on [5,7] is identical to [1,3] with g(5)=0:
g(x)=0x=7.
Note that g(7)=0.

4. For the interval t[7,8]:
Since f(t) is linear from 2 at t=7 to 2 at t=9, we have f(t)<0 for all t[7,8) and f(8)=0.
Thus, the integral g(x)=g(7)+7xf(t)dt=7xf(t)dt is strictly decreasing and negative for x(7,8].
Therefore, there are no solutions to g(x)=0 in the interval (7,8].

Combining the solutions in the interval (1,8], we find exactly three values: x=3, x=5, and x=7.
Thus, the number of solutions is:
α=3

Finally, we calculate the sum α+β:
α+β=3+2=5

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