Mình làm trước nhé!
2. Đặt y = 1 - x, ta có: [tex](1 - x)tan \frac{\pi.x}{2} = y.tan(\frac{\pi}{2} - \frac{\pi.y}{2}) = y.cot \frac{\pi.y}{2} = y. cos(y\pi)/2 : sin(y\pi)/2[/tex]
Khi x--> 0 thì: [tex]cos (\pi.y)/2 = 1 - \frac{1}{2}(\frac{{\pi.y}}{2})^{2}, sin \frac{{\pi.y}}{2} = \frac{{\pi.y}}{2}[/tex]