grandes-ecoles 2020 Q16

grandes-ecoles · France · x-ens-maths-b__mp_cpge Reduction Formulae Derive a Reduction/Recurrence Formula via Integration by Parts
We admit the identities: $$\lim_{a \rightarrow +\infty} \int_0^a \sin(x^2) \mathrm{d}x = \lim_{a \rightarrow +\infty} \int_0^a \cos(x^2) \mathrm{d}x = \frac{\sqrt{2\pi}}{4}$$
Show that there exist real numbers $c, c' \in \mathbb{R}$ such that, as $a \rightarrow +\infty$, $$\int_0^a \sin(x^2) \mathrm{d}x = \frac{\sqrt{2\pi}}{4} + \frac{c}{a} \cos(a^2) + \frac{c'}{a^3} \sin(a^2) + O\left(\frac{1}{a^5}\right)$$
We admit the identities:
$$\lim_{a \rightarrow +\infty} \int_0^a \sin(x^2) \mathrm{d}x = \lim_{a \rightarrow +\infty} \int_0^a \cos(x^2) \mathrm{d}x = \frac{\sqrt{2\pi}}{4}$$

Show that there exist real numbers $c, c' \in \mathbb{R}$ such that, as $a \rightarrow +\infty$,
$$\int_0^a \sin(x^2) \mathrm{d}x = \frac{\sqrt{2\pi}}{4} + \frac{c}{a} \cos(a^2) + \frac{c'}{a^3} \sin(a^2) + O\left(\frac{1}{a^5}\right)$$