Think & Learn / Byju's — worst chat with subject matter expert

I am a student of class 11. I have just started my mathematics subject. I had a doubt in the 1st chapter 'sets'. I thought of chatting with the subject matter expert for help. First of all there were many delays in replies which i can understand might be due to connection problem. But the expert named rohit didn't answered correctly in the beginning even once. He gave answers on other things. Even before i had asked the doubts, he was ready to send me the links related to my doubts as if he had read my mind before only. After much suffering he gave me the answer of my doubt. The answer he gave me contradicted what was written in the book and i told this to him. He said the book has been printed wrong i asked him once more if he was sure that the book was wrong and he said yes. Then i rated him good and closed the chat because i thought how could subject matter experts be wrong but today it turned out that he was wrong as some other subject matter expert explained me how the book was right.

My doubt was if a and b are two disjoint sets then is their union null?
Because if it was null then this formula didn't hold true:
N (A u b)=n (A) + n (B) - n (A n b)

To this rohit told me that union of disjoint sets is null and so the book is wrote if it says that the formula is true for disjoint sets also.

Today another expert explained me that the book is right as the union of disjoint sets is not null.

This incidence has truly removed my trust over byjus' experts which i thought would help me in 11th. Please take some action against this.