Is this your chair? [ ]

A. It's your desk.

B. It's my desk.

C. Yes, it is.

观察下列等式：

（第1条）3²+4²=5²

（第2条）10²+11²+12²=13²+14²

（第3条）21²+22²+23²+24²=25²+26²+27²

写出（第4条）______．