1.The math problem is so difficult that I can't work it out.

1.The math problem is so difficult that I can't work it out.(改为简单句) The math problem is ______
1.The math problem is so difficult that I can't work it out.(改为简单句)
The math problem is _______ difficult _______ _______ _______ ______ out.
2.The doctor died ten days ago.(改为现在完成时态)
The doctor _______ ________ _______ _______ ten years ago,
3.People use Chinese as the first language in China.(改为同义句)
Chinese _______ _______ ________ the first language in China.