Answer :

The first terms of each line are 4,7,13,22…


The difference between each consecutive term is


7-4 = 3


13-7 = 6


22-13 = 9


i.e. 3,6,9… in AP


For 4th line,


First term = 22 + 12 = 34 , d = 3


For 5th line


First term = 34 + 15 = 49 , d = 3


Also from observation, nth line has n terms


Hence, 4th line


34,37,40,43


And 5th line


49,52,55,58,61


The sequence of common difference is


The difference between the first terms of each line forms an AP which is


3,6,9…


For first term of 20th line, sum of 19 terms of above sequence to be added to first term of first line.


Sum of 19 terms =


= 570


For 20th line,


First term = 4 + 570 = 574


It has 20 terms


Last term = 574 + 19×3 = 574 + 57 = 631


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