Arithmetic Progression is a sequence whose terms increase or decrease by a fixed number. This fixed number is called the common difference. If ‘a’ is the first term & ‘d’ is a common difference, then Arithmetic Progression can be written as a, a + d, a + 2d, ..., a + (n – 1) d, ...
We can find nth term of an Arithmetic Progression as Tn=a+(n−1)d where d = Tn−Tn−1
The Sum of the first n terms : Sn=2n[2a+(n−1)d]orSn=2n[a+l] where l is the last term.
We can also write nth term as Tn=Sn−Sn−1
ThingstoNote:
nth term of an Arithmetic Progression is of the form An + B, i.e. a linear expression in n, in such case, the coefficient of n is the common difference of the A.P., i.e. A
Sum of the first n terms of an Arithmetic Progression is An2 + Bn, i.e., a quadratic expression in n. In such case, the common difference is twice the coefficient of n2, i.e., 2A
Three numbers in Arithmetic Progression can be taken as a–d,a,a+d;
Four numbers in Arithmetic Progression can be taken as a–3d,a–d,a+d,a+3d
Five numbers in Arithmetic Progression are a–2d,a–d,a,a+d,a+2d
Six terms in Arithmetic Progression are a–5d,a–3d,a–d,a+d,a+3d,a+5d etc.
If a, b, c are in Arithmetic Progression, then b = 2a+c
If a1,a2,a3,.... and b1,b2,b3,..... are two A.P.s, the sum and difference of these Arithmetic Progression is (a1±b1,a2±b2,a3±b3,.....) are also in Arithmetic Progression.
Any term of an Arithmetic Progression (except the first & last) equals half the sum of terms that are equidistant from it. Tr=2Tr−k+Tr+k, k < r
If each term of an Arithmetic Progression is increased or decreased by the same number, then the resulting sequence is also an Arithmetic Progression having the same common difference.
If each term of an Arithmetic Progression is multiplied or divided by the same nonzero number (k), then the resulting sequence is also an Arithmetic Progression whose common difference is kd & kd respectively, where d is the common difference of original Arithmetic Progression.
2. Geometric Progression (GP):
Geometric Progression is a sequence of numbers whose first term is non-zero & each of the succeeding terms equals the preceding terms multiplied by a constant. Thus, the ratio of successive terms is constant in a Geometric Progression.
This constant factor is called the common ratio of the series & is obtained by dividing any term by the immediately previous term. Therefore a,ar,ar2,ar3,ar4,..... is a Geometric Progression with 'a' as the first term & 'r' as a common ratio.
We can find nth term of an Geometric Progression as Tn=arn−1
Sum of the first n terms Sn=(r−1)a(rn−1),ifr=1
If a, b, c are in Geometric Progression ⇒b2=ac⇒ loga, logb, logc, are in Arithmetic Progression.
ThingstoNote:
In a Geometric Progression, the product of the kth term from the beginning and the kth term from the last is always constant, equal to the product of the first and last term.
Three numbers in Geometric Progression : ra,a,ar
Four numbers in Geometric Progression : r3a,ra,ar,ar3
Five numbers in Geometric Progression : r2a,ra,a,ar,ar2
Six numbers in Geometric Progression : r5a,r3a,ra,ar,ar3,ar5
If each term of a Geometric Progression is raised to the same power, then the resulting series is also a Geometric Progression.
If each term of a Geometric Progression is multiplied or divided by the same non-zero quantity, then the resulting sequence is also a Geometric Progression.
If a1,a2,a3,... and b1,b2,b3,... be two geometric progressions of common ratio r1 and r2 respectively, then a1b1,a2b2,... and b1a1,b2a2,b3a3,... will also form a Geometric Progression whose common ratio will be r1,r2 and r2r1 respectively.
In a positive Geometric Progression, every term (except the first) is equal to the square root of the product of its two terms, which are equidistant from it, i.e., Tr=Tr−kTr+k, k < r
If a1,a2,a3,...,an is a Geometric Progression of nonzero, nonnegative terms then loga1,loga2,loga2,...,logan is an Arithmetic progression and vice-versa.
3. Harmonic Progression (HP):
A sequence is said to be in Harmonic Progression if the reciprocals of its terms are in Arithmetic Progression. If the sequence a1,a2,a3,...,an is a Harmonic Progression then a11,a21,...,an1 Arithmetic Progression & vice-versa.
Here, we have the formula to find the nth term but do not have the formula for the sum of the first n terms of a Harmonic Progression.
The general form of a harmonic progression is a1,a+d1,a+2d1,.....,a+(n−1)d1
ThingstoNote:
No term of any Harmonic Progression can be zero.
If a, b, c are in Harmonic Progression ⇒b=a+c2ac or ca=b−ca−b
4. Arithmetic Mean (AM):
If three terms are in Arithmetic Progression, then the middle term is the Arithmetic Mean between the other two, so if a, b, and c are in Arithmetic Progression, b is AM of a & c.
n arithmetic means between two numbers:
If a, b are any two given numbers & a, A1,A2,....,An,b are in Arithmetic progression then A1,A2,....,An are the n AM’s between a & b, then A1=a+d,A2=a+2d,....,An=a+nd where d=n+1b−a
ThingstoNote:
Sum of n AM's inserted between a & b is equal to n times the single AM between a & b, i.e.
r=1∑nAr=nA
where A is the single AM between a & b i.e., 2a+b
5. Geometric Mean (GM):
If a, b, and c are in Geometric Progression, then b is the Geometric Mean between a & c i.e., b2 = ac, therefore b = ac
n Geometric Mean (GM) means between two numbers:
If a, b are any two given positive numbers & a, G1,G2,....,Gn,b are in Geometric progression then G1,G2,....,Gn are the n GM’s between a & b, then G1=ar,G2=ar2,....,Gn=arn where r=(ab)n+11
ThingstoNote:
The product of n GMs between a & b is equal to nth power of the single GM between a & b i.e. Πr=1nGr=(G)n where G is the single GM between a & b i.e. ab
6. Harmonic Mean (HM):
If a, b, and c are in HP, then b is HM between a & c, then b=a+c2ac
ThingstoNote:
If A, G, and H are, respectively, AM, GM, and HM between two positive numbers a & b, then (i)
G2=AH (A, G, H constitute a GP)
A≥G≥H
A=G=H⟺a=b
(ii)
Let a1,a2,a3,...,an be n positive real numbers, then we define their
Arithmetic mean as A = na1+a2+...+an
Geometric mean as G = (a1.a2...an)n1
Harmonic mean as H = a11+a21+...+an1n
It can be shown that A≥G≥H Moreover, equality holds at either place ⟺a1=a2=...=an
7. Arithmetic-Geometric Progression (AGP):
Sum of First n terms of an Arithmetic-Geometric Progression
Let Sn=a+(a+d)r+(a+2d)r2+....+[a+(n−1)d]rn−1
then Sn=1−ra+(1−r)2dr(1−rn−1)−1−r[a+(n−1)d]rn−1,r=1 Sum to Infinity
If |r| < 1 and n→∞ then Limn→∞rn=0⇒S∞=1−ra+(1−r)2dr
8. Properties of Sigma Notations:
(a)∑r=1n(ar±br)=∑r=1nar±∑r=1nbr (a)∑r=1nkar=k∑r=1nar (a)∑r=1nk=nk; where k is a constant.