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Consider the equation
$$\int_1^e \frac{(\log_e x)^{1/2}}{x\left(a - (\log_e x)^{3/2}\right)^2} dx = 1, \quad a \in (-\infty, 0) \cup (1, \infty).$$
Which of the following statements is/are TRUE?
Write $$\ln x$$ instead of $$\log_e x$$ to shorten the symbols.
The equation is
$$\int_{1}^{e} \frac{(\ln x)^{1/2}}{x\left(a-(\ln x)^{3/2}\right)^2}\,dx = 1,$$
with $$a \in (-\infty,0)\cup(1,\infty).$$
Step 1 : Suitable substitution
Let$$t=(\ln x)^{3/2}\,.$$
Then $$\ln x = t^{2/3}\quad\Longrightarrow\quad x = e^{\,t^{2/3}}$$ and hence $$dx = e^{\,t^{2/3}}\cdot\frac{2}{3}\,t^{-1/3}\,dt.$$ (We used $$\frac{d}{dt}\bigl(t^{2/3}\bigr)=\frac{2}{3}t^{-1/3}$$.)
Step 2 : Transform the integrand
The numerator becomes
$$(\ln x)^{1/2}=t^{1/3},$$
while the denominator equals
$$x\bigl(a-(\ln x)^{3/2}\bigr)^2=e^{\,t^{2/3}}(a-t)^2.$$
Therefore
$$\frac{(\ln x)^{1/2}}{x\left(a-(\ln x)^{3/2}\right)^2}dx
=\frac{t^{1/3}}{e^{\,t^{2/3}}(a-t)^2}\;\Bigl[e^{\,t^{2/3}}\cdot\frac{2}{3}t^{-1/3}\,dt\Bigr]
=\frac{2}{3}\,\frac{1}{(a-t)^2}\,dt.$$
The factor $$e^{\,t^{2/3}}$$ cancels out completely.
Step 3 : New limits
When $$x=1,\; \ln 1 =0\;\Longrightarrow\; t=0.$|
When $$x=e,\; $$\ln$$ e =1\;\Longrightarrow\; t=1.$$
Hence the original integral reduces to $$$$\frac{2}{3}$$$$\int_{0}^{1}\frac{dt}{(a-t$$)^2}=1.$$
Step 4 : Evaluate the simple integral
Recall
$$$$\int$$$$\frac{dt}{(a-t)^2}=\frac{1}{a-t}$$+C,$$
since $$$$\frac{d}{dt}$$\Bigl($$\frac{1}{a-t}$$\Bigr)=$$\frac{1}{(a-t)^2}$$.$$
Thus
$$$$\int_{0}^{1}\frac{dt}{(a-t$$)^2}
=\Bigl[$$\frac{1}{a-t}$$\Bigr]_{0}^{1}
=$$\frac{1}{a-1}-\frac{1}{a}$$.$$
Therefore
$$$$\frac{2}{3}$$$$\left$$($$\frac{1}{a-1}-\frac{1}{a}$$$$\right$$)=1.$$
Step 5 : Solve for $$a$$
Combine the fractions:
$$$$\frac{1}{a-1}-\frac{1}{a}=\frac{a-(a-1)}{a(a-1)}=\frac{1}{a(a-1)}$$.$$
Insert this into the equation:
$$$$\frac{2}{3}$$$$\cdot$$$$\frac{1}{a(a-1)}$$=1
\;\Longrightarrow\;
$$\frac{2}{3a(a-1)}$$=1
\;\Longrightarrow\;
2=3a(a-1).$$
Expand:
$$3a^{2}-3a-2=0.$$
Solve the quadratic:
$$a=$$\frac{3\pm\sqrt{9+24}$$}{6}
=$$\frac{3\pm\sqrt{33}$$}{6}.$$
Step 6 : Check domain
Compute the two roots approximately:
$$a_{1}=$$\frac{3+\sqrt{33}$$}{6}$$\approx$$$$\frac{3+5.7446}{6}$$$$\approx$$1.4574,$$
$$a_{2}=$$\frac{3-\sqrt{33}$$}{6}$$\approx$$$$\frac{3-5.7446}{6}$$$$\approx$$-0.4574.$$
Thus
$$a_{1}$$\in$$(1,$$\infty$$),\qquad a_{2}$$\in$$(-$$\infty$$,0),$$
exactly the allowed intervals.
Step 7 : Nature and count of the solutions
Both $$a_{1},a_{2}$$ contain $$$$\sqrt{33}$$,$$ an irrational number, so both are irrational.
No integer satisfies the equation, and there are **two** (more than one) admissible values.
Therefore the true statements are:
Option C: An irrational number satisfies the equation. (In fact both solutions are irrational.)
Option D: More than one $$a$$ satisfies the equation.
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