Should i study full rs aggrawal book for CLAT 2019 maths section or only few chapters are important.

Hi Yadnyesh.
For CLAT you need to study mathematics as per class 10 standard. You need not do the whole book, you can cover the following topics as per the Syllabus of CLAT and previous year questions-
ARITHEMATIC:
Number system, Square roots, Surds and indices, H.C.F and L.C.M, Simplification Ratio and proportions, Average Logarithms and Functions, Fractions and decimals,Time and work
COMMERCIAL MATHS:
Interest, Percentage, Profit, loss and Discounts, Partnership.
MENSURATION
Areas, Volumes
MODERN MATHEMATICS
Permutation & Combination, Probability ,Clocks & Calendars
DATA INTERPRETATION
Best of luck.