Soient \(J(x) = \int _{t=0}^{\pi /2}\dfrac{\mathrm{ \;d}t}{\sqrt {\sin^2 t + x^2 \cos^2 t}}\) et \(K(x) = \int _{t=0}^{\pi /2}\dfrac{\cos t\,\mathrm{ \;d}t}{\sqrt {\sin^2 t + x^2 \cos^2 t}}\).
Calculer \(\lim_{x\to 0_{+} }(J(x)-K(x))\) et montrer que \(J(x) = -\ln x + 2\ln2 + o_{x\to 0_{+} }(1)\).