Cho hàm số \(f(x) = \left\{ \begin{array}{l} 1 - {x^2}\,\,\,{\rm{khi }}x \le 1\\ 2x - 2\,\,\,\,\,{\rm{khi }}x > 1 \end{array} \right.\). Tính tích phân \(\int\limits_{-\frac{\pi }{2}}^{\frac{\pi }{4}}{f\left( 5\sin 2x-1 \right)}\cos 2x\text{d}x\).
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Lời giải:
Báo saiXét \(I=\int\limits_{-\frac{\pi }{2}}^{\frac{\pi }{4}}{f\left( 5\sin 2x-1 \right)\cos 2x\text{d}x}\)
Đặt \(5\sin 2x-1=t\)\(\Rightarrow 10\cos 2x\text{d}x=\text{d}t\Rightarrow \cos 2x\text{d}x=\frac{1}{10}\text{d}t\)
Với \(x=-\frac{\pi }{2}\)\(\Rightarrow \)\(t=-1\)
\(x=\frac{\pi }{4}\)\(\Rightarrow \)\(t=4\)
\(\Rightarrow I=\frac{1}{10}\int\limits_{-1}^{4}{f\left( t \right)\text{d}t}=\frac{1}{10}\int\limits_{-1}^{4}{f\left( x \right)\text{d}x}=\frac{1}{10}\int\limits_{-1}^{1}{f(x)\text{d}x}+\frac{1}{10}\int\limits_{1}^{4}{f(x)\text{d}x}\)
\(=\frac{1}{10}\int\limits_{-1}^{1}{\left( 1-{{x}^{2}} \right)\text{d}x}+\frac{1}{10}\int\limits_{1}^{4}{\left( 2x-2 \right)\text{d}x}=\frac{31}{30}.\)