Tính các tích phân sau:
a) \(\int\limits^4_1\left(x^3-2\sqrt{x}\right)dx\); b) \(\int\limits^{\dfrac{\pi}{2}}_0\left(\cos x-\sin x\right)dx\); c) \(\int\limits^{\dfrac{\pi}{4}}_{\dfrac{\pi}{6}}\dfrac{dx}{\sin^2x}\); d) \(\int\limits^{16}_1\dfrac{x-1}{\sqrt{x}}dx\).