Compare the two definite integrals \( I_1=\int\limits_{-1}^1\left(x+\frac{\pi}2\right)\mathrm{d}x \) and \( I_2=\int\limits_0^{\frac{\pi}4}\mathrm{tg}\,x\cdot\cos x\,\mathrm{d}x \).
Evaluate the definite integral \( \int\limits_0^{\frac{\pi}6}\frac{3\cos2t}{\cos t+\sin t}\,\mathrm{d}t \). Which of the following intervals contains the value of the integral?