MOST IMPORTANT

Question 1

Remember that the + and - signs separate the terms. So when you are taking something common, you must take it across those + and -signs

Answer 1

For eg: 81×80×79! - 80×79! + 79!/79!
IMP: Consider 79! as 1×79!
Hence it becomes:
79! (81×80 - 80 + 1)/79!