作品工作 (about my work today)

我是("一个" is optional here)数学家. 今天我试着证明一个定理. 它是很难.

"作品" means a play, novel, painting, piece of music etc. that is produced by you.

Therefore, the Chinese for "Shakespeare's works" is "莎士比亚的作品", while you can't refer to your work today as "作品".

As for "它是难", it is grammatically correct per se, but it is incorrect to use it in that context. For more detailed explanation, you can read a notebook entry of mine and an answer I gave to a question on these two pages:

http://www.italki.com/entry/162678

http://www.italki.com/entry/347334

作品 (about my work today)

我是(一位 attributive here more formal,or you could omitt it according to the situation)数学家. 今天我试（着 more local ）证明一条（like a/an in english）定理. 它是难. （这条定理很难。In Chinese, we used to repeat but rarely use pronoun "it" . If you want to translate "it is difficult/beautiful/easy..."we just say Sth 很难/漂亮/简单......）

作品 (about my work today)

我是一個数学家. 今天我试证明定理. 我覺得它很难.

作品 (about my work today)

我是数学家. 今天我试着证明一个定理. 它是很难.

作品 (about my work today)

我是一位数学家. 今天我试着证明一个定理. 它是很难.