71.
If Sn denotes the sum of the first r terms of an A.P. Then, S3n : (S2n – Sn) is

72.
If $$\frac{1}{{x + 2}},$$  $$\frac{1}{{x + 3}},$$  $$\frac{1}{{x + 5}}$$   are in A.P. then x = ?

73.
If the first term of an A.P. is a and nth term is b, then its common difference is

74.
If $$\frac{{5 + 9 + 13 + ...\,{\text{to}}\,n\,{\text{terms}}}}{{7 + 9 + 11 + ...\,{\text{to}}\,\left( {n + 1} \right)\,{\text{terms}}}}$$       $$ = \frac{{17}}{{16}},$$  then n = ?

75.
The common difference of the A.P. is $$\frac{1}{{2q}},$$ $$\frac{{1 - 2q}}{{2q}},$$  $$\frac{{1 - 4q}}{{2q}},$$  . . . . is

76.
The first three terms of an A.P. respectively are 3y – 1, 3y + 5 and 5y + 1. Then, y equals

77.
If 7th and 13th terms of an A.P. be 34 and 64 respectively, then its 18th term is

78.
If the sum of it terms of an A.P. is 2n2 + 5n, then its nth term is

79.
If the sum of first n even natural number is equal to k times the sum of first n odd natural numbers, then k =

80.
In an AP, Sp = q, Sq = p and S denotes the sum of first r terms. Then, Sp+q is equal to