$\displaystyle\int_{\pi/6}^{\pi/3} \dfrac{dx}{1+\sqrt{\tan x}}$ equals
A$\dfrac{\pi}{6}$
B$\dfrac{\pi}{3}$
C$\dfrac{\pi}{12}$
D$\dfrac{\pi}{4}$
Answer & Solution
Correct answer: C. $\dfrac{\pi}{12}$
1. Write $I=\int_{\pi/6}^{\pi/3}\dfrac{\sqrt{\cos x}}{\sqrt{\cos x}+\sqrt{\sin x}}\,dx$.
2. By $\mathbf{P_3}$ with $a+b=\tfrac{\pi}{2}$, replace $x$: numerator becomes $\sqrt{\sin x}$.
3. Adding: $2I=\int_{\pi/6}^{\pi/3} dx=\dfrac{\pi}{3}-\dfrac{\pi}{6}=\dfrac{\pi}{6}$.
4. Hence $I=\dfrac{\pi}{12}$.
_Source: syllabus Mathematics Ch 7 "Integrals", p.345_
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