Cho \(\int\limits^6_0f\left(x\right)\text{d}x=12\). Tích phân \(\int\limits^2_0f\left(3x\right)\text{d}x\) bằng
\(6\).\(36\).\(2\).\(4\).Hướng dẫn giải:Cách 1: Đặt \(t=3x\) thì \(I=\int\limits^6_0f\left(t\right).\frac{1}{3}\text{d}t=\frac{1}{3}\int\limits^6_0f\left(t\right)\text{d}t=\frac{1}{3}\int\limits^6_0f\left(x\right)\text{d}x=\frac{1}{3}.12=4.\)
Cách 2: Có \(\int\limits^6_0\text{d}x=6\Rightarrow\int\limits^6_02\text{d}x=12\Rightarrow f\left(x\right)=2\Rightarrow f\left(3x\right)=2\Rightarrow\)\(\int\limits^2_0f\left(3x\right)\text{d}x=\int\limits^2_02\text{d}x=2.2=4.\)
