Ta có:\(\int\limits_0^{2008\pi } {\sin (2008x + \sin x)dx} = \int\limits_0^{2008\pi } {\sin (2008x)\cos (\sin x) + \cos (2008x)\sin (\sin x)dx} \)
Đặt f(x) = sin(sin(x)), ta có f(-x) = sin(sin(-x)) = sin(-sin(x)) = -sin(sin(x)) = -f(x). Vậy f(x) là hàm lẻ.
Đặt g(x) = cos(sin(x)), ta có g(-x) = cos(sin(-x)) = cos(-sin(x)) = cos(sin(x)) = g(x). Vậy g(x) là hàm chẵn.
Ta có:\(\int\limits_0^{2008\pi } {\sin (2008x + \sin x)dx} = \int\limits_0^{2008\pi } {\sin (2008x)\cos (\sin x)dx} + \int\limits_0^{2008\pi } {\cos (2008x)\sin (\sin x)dx} \)
Xét tích phân:\(I = \int\limits_0^{2008\pi } {\cos (2008x)\sin (\sin x)dx} \). Đặt t = 2008π - x. Khi đó dt = -dx, x = 0 ⇒ t = 2008π, x = 2008π ⇒ t = 0.
Do đó:\(I = \int\limits_{2008\pi }^0 {\cos (2008(2008\pi - t))\sin (\sin (2008\pi - t))( - dt)} = \int\limits_0^{2008\pi } {\cos (2008(2008\pi - t))\sin (\sin (2008\pi - t))dt} \)
Vì cos(2008(2008π - t)) = cos(2008.2008π - 2008t) = cos(-2008t) = cos(2008t)
và sin(sin(2008π - t)) = sin(sin(-t)) = sin(-sin(t)) = -sin(sin(t))
Nên:\(I = \int\limits_0^{2008\pi } {\cos (2008t)( - \sin (\sin t))dt} = - \int\limits_0^{2008\pi } {\cos (2008t)\sin (\sin t)dt} = - I\)
Suy ra I = 0.
Xét tích phân:\(J = \int\limits_0^{2008\pi } {\sin (2008x)\cos (\sin x)dx} \)
Đặt x = u + π. Khi đó dx = du, x = 0 ⇒ u = -π, x = 2008π ⇒ u = 2007π.
Suy ra:\(J = \int\limits_{ - \pi }^{2007\pi } {\sin (2008(u + \pi ))\cos (\sin (u + \pi ))du} = \int\limits_{ - \pi }^{2007\pi } {\sin (2008u + 2008\pi )\cos ( - \sin u)du} \)
Vì sin(2008u + 2008π) = sin(2008u)cos(2008π) + cos(2008u)sin(2008π) = sin(2008u)
và cos(-sin(u)) = cos(sin(u))
Nên:\(J = \int\limits_{ - \pi }^{2007\pi } {\sin (2008u)\cos (\sin u)du} \)
Ta có:\(\int\limits_{ - \pi }^{2007\pi } {\sin (2008u)\cos (\sin u)du} = \int\limits_{ - \pi }^0 {\sin (2008u)\cos (\sin u)du} + \int\limits_0^{2007\pi } {\sin (2008u)\cos (\sin u)du} \)
Xét:\(\int\limits_{ - \pi }^0 {\sin (2008u)\cos (\sin u)du} \). Đặt t = -u. Khi đó dt = -du, u = -π ⇒ t = π, u = 0 ⇒ t = 0.
Suy ra:\(\int\limits_{ - \pi }^0 {\sin (2008u)\cos (\sin u)du} = \int\limits_{\pi }^0 {\sin ( - 2008t)\cos (\sin ( - t))( - dt)} = \int\limits_{\pi }^0 {\sin ( - 2008t)\cos ( - \sin (t))( - dt)} \)
Vì sin(-2008t) = -sin(2008t) và cos(-sin(t)) = cos(sin(t))
Nên:\(\int\limits_{\pi }^0 {\sin ( - 2008t)\cos ( - \sin (t))( - dt)} = - \int\limits_0^{\pi } {\sin (2008t)\cos (\sin t)dt} \)
Do đó:\(J = - \int\limits_0^{\pi } {\sin (2008u)\cos (\sin u)du} + \int\limits_0^{2007\pi } {\sin (2008u)\cos (\sin u)du} = \int\limits_0^{2007\pi } {\sin (2008u)\cos (\sin u)du} - \int\limits_0^{\pi } {\sin (2008u)\cos (\sin u)du} \)
Ta có:\(\int\limits_0^{2007\pi } {\sin (2008u)\cos (\sin u)du} = \int\limits_0^{\pi } {\sin (2008u)\cos (\sin u)du} + \int\limits_{\pi }^{2\pi } {\sin (2008u)\cos (\sin u)du} + ... + \int\limits_{2006\pi }^{2007\pi } {\sin (2008u)\cos (\sin u)du} \)
Xét:\(\int\limits_{k\pi }^{(k + 1)\pi } {\sin (2008u)\cos (\sin u)du} \). Đặt u = t + kπ. Khi đó du = dt, u = kπ ⇒ t = 0, u = (k+1)π ⇒ t = π.
Suy ra:\(\int\limits_{k\pi }^{(k + 1)\pi } {\sin (2008u)\cos (\sin u)du} = \int\limits_0^{\pi } {\sin (2008(t + k\pi ))\cos (\sin (t + k\pi ))dt} = \int\limits_0^{\pi } {\sin (2008t + 2008k\pi )\cos (\sin t\cos k\pi + \cos t\sin k\pi )dt} \)
Vì sin(2008t + 2008kπ) = sin(2008t)cos(2008kπ) + cos(2008t)sin(2008kπ) = sin(2008t)cos(2008kπ) = sin(2008t)
Nên:\(\int\limits_{k\pi }^{(k + 1)\pi } {\sin (2008u)\cos (\sin u)du} = \int\limits_0^{\pi } {\sin (2008t)\cos (\sin (t + k\pi ))dt} \)
Do đó:\(J = 0\)
Vậy:\(\int\limits_0^{2008\pi } {\sin (2008x + \sin )dx} = 0\)
