(13) Answer is (d) 61
(1 – 3x + 3x2 – x3)20 = [(1 – x)3 ]20 = (1 – x)60
Therefore number of dissimilar terms in the expansion of (1 – 3x + 3x2 –x3)20 is 61
(14) Answer is (a) 501
The possible set of values of r is {0, 2, 4, ............, 1000}
Hence, number of rational terms is 501
(15)