ece205: fix bug

docs/2a/ece205.md

@@ -396,7 +396,7 @@ Thus if a Fourier series on $(0,L)$ exists, it can be expressed as either a **Fo
     
     \begin{align*}
     u(x,t)&=\sum^\infty_{n=1}\alpha_ne^{-left(\frac{n\pi\sqrt 2}{\pi}\right)^2t}\sin(\frac{n\pi x}{\pi}) 
-    &=\sum^\infty_{n=1}\apha_ne^{-2n^2t}\sin(nx) \\
+    &=\sum^\infty_{n=1}\alpha_ne^{-2n^2t}\sin(nx) \\
     \alpha_n&=\frac 2 L\int^L_0f(x)\sin(\frac{n\pi x}{L})dx \\
     &=\frac2\pi\int^{\pi/2}_0\frac\pi 2\sin(nx)dx+\frac2\pi\int^\pi_{\pi/2}(x-\frac\pi2\sin(nx)dx \\
     &=\frac 1 n[1+(-1)^{n+1}-\cos(\frac{n\pi}{2})-\frac{2}{n\pi}\sin(\frac{n\pi}{2}]