代码如下：
Subscript[h, 0] = 5; theta0 = Pi/4; Subscript[c, 0] = 1;
Subscript[l, 0] = Subscript[h, 0]/Sin[theta0];
Subscript[l, 1] = Sqrt[((Subscript[l, 0]/2)^2) + (Subscript[c, 0]^2) ];
a = Subscript[h, 0] /Tan[theta0];
h = Subscript[h, 0] - x;
theta01 = ArcTan[h/a];
Subscript[l, 01] = h/Sin[theta01];
thetamid = ArcCos[Subscript[l, 01]/(2*Subscript[l, 1] )]

不用下标就行。

感谢感谢，在此之后我又遇到一个画图的问题，函数出来了，但是图像却没有，好像是函数定义有问题，但是不太清楚问题出在哪。
代码如下：
Clear[h0 , theta0 , c0, m];
h0 = 5; theta0 = Pi/4; c0 = 1; m = 1;
l0 = h0/Sin[theta0];
l1 = Sqrt[((l0/2)^2) + (c0^2) ];
a = h0 /Tan[theta0];
h = h0 - x;
theta01 = ArcTan[h/a];
l01 = h/Sin[theta01];
thetamid = ArcCos[l01/(2*l1 )];
theta11 = thetamid + theta01;
theta21 = Pi - 2 (theta11 - theta01);
theta31 = Pi/2 - theta01 + (theta11 - theta01);
dtheta11 = D[theta11, x];
dtheta21 = D[theta21, x];
dtheta31 = D[theta31, x];
f = m * Abs[dtheta11] + m * Abs[dtheta21] + m * Abs[dtheta31];
f1 = Simplify[f]
F[x_] = Function[f1, x]
Plot[F[x], {x, 0, 10}]