🐛 Fix render

lib/solver.dart

@@ -316,12 +316,12 @@ ${a2}x ${b2 >= 0 ? '+' : ''} ${b2}y = $c2 & (2)
         explanation: '为了消去变量 y，将方程(1)两边乘以 $b2，方程(2)两边乘以 $b1。',
         formula:
             '''
-
+\$\$
 \\begin{cases}
-${newA1}x ${b1 * b2 >= 0 ? '+' : ''} ${b1 * b2}y = $newC1 & (3) \\
+${newA1}x ${b1 * b2 >= 0 ? '+' : ''} ${b1 * b2}y = $newC1 & (3) \\\\
 ${newA2}x ${b1 * b2 >= 0 ? '+' : ''} ${b1 * b2}y = $newC2 & (4)
 \\end{cases}
-
+\$\$
 ''',
       ),
     );
@@ -360,14 +360,14 @@ ${newA2}x ${b1 * b2 >= 0 ? '+' : ''} ${b1 * b2}y = $newC2 & (4)
           explanation: '将 x = $x 代入原方程(2)中。',
           formula:
               '''
-
+\$\$
 \\begin{aligned}
 $a2($x) + ${b2}y &= $c2 \\\\
 ${a2 * x} + ${b2}y &= $c2 \\\\
 ${b2}y &= $c2 - ${a2 * x} \\\\
 ${b2}y &= ${c2 - a2 * x}
 \\end{aligned}
-
+\$\$
 ''',
         ),
       );
@@ -394,14 +394,14 @@ ${b2}y &= ${c2 - a2 * x}
           explanation: '将 x = $x 代入原方程(1)中。',
           formula:
               '''
-
+\$\$
 \\begin{aligned}
 $a1($x) + ${b1}y &= $c1 \\\\
 ${a1 * x} + ${b1}y &= $c1 \\\\
 ${b1}y &= $c1 - ${a1 * x} \\\\
 ${b1}y &= ${c1 - a1 * x}
 \\end{aligned}
-
+\$\$
 ''',
         ),
       );