add a solution for M^2=I

%% Cell type:code id: tags:

``` python

``` 

from sympy import *

x = symbols ('x')

f = symbols('f', cls=Function)

dsolve(f(x).diff(x)- cos(f(x)), f(x))

```

%% Output

    [Eq(f(x), pi - asin(1/tanh(C1 + x))), Eq(f(x), asin(1/tanh(C1 + x)))]

%% Cell type:code id: tags:

``` python

``` 

# 计算所有满足 M^2=I 的整系数矩阵, 无法使用这一方法计算 M^3

from sympy import *

a, b, c, d = symbols('a b c d');

M=Matrix([[a,b],[c,d]]);

K=M*M

N=Matrix([[1,0],[0,1]]);

solve(Eq(K,N),(a,b,c,d))

```

%% Output

    [(-1, 0, 0, -1),

     (-1, 0, 0, 1),

     (1, 0, 0, -1),

     (1, 0, 0, 1),

     (-d, (1 - d**2)/c, c, d)]

%% Cell type:code id: tags:

``` 

```