if I prepare only important questions of 14 chapters in phy and prepare 10 sample papers of arihanth including2019 will i be able to get 90 and above I am CBSE i am in class12

Gautham, See, it is very important to go through the previous years and sample question papers and analyse them to know about the important topics and the pattern of exam but you can't just rely on the previous year papers or important questions of all the chapters to get you 90+ marks in the Physics Board exams. Although, there are some questions in the board exams which have some inclination from the previous year papers or the sample papers but they aren't the same and so you require to understand the concepts and theory behind it to solve it. Also, many questions are framed directly from the application of theory in the NCERT textbooks as evident from the previous few years papers, so just solving the sample papers and the important questions would not suffice. Yes, it may help you get a few questions right but if you are aiming for 90+ marks ie 62+ marks in theory (assuming that you got close to 28 in practicals), you would have to be thorough with the NCERT Theory and should have at least read it 2-3 times. If you are thorough with NCERT textbook and have understood its concepts, then the sample papers or the previous 10 years papers should be sufficient to get you 65+ marks as the exam isn't as tough as it seems but it requires the student to be clear about the concepts and theory and also its application and you can't probably understand the full concept from the sample papers.