SEQUENCE & SERIES 1. ARITHMETIC PROGRESSION (AP) : AP is 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 the common difference, then AP can be written as a, a + d, a + 2d, ..., a + (n – 1) d, ... (a) \(n^{\text {th }}\) term of this AP \(\boxed{T_{n}=a+(n-1) d}\) , where \(d=T_{n}-T_{n-1}\) (b) The sum of the first \(n\) terms : \(\boxed{S_{n}=\frac{n}{2}[2 a+(n-1) d]=\frac{n}{2}[a+\ell]}\) , where \(\ell\) is the last term. (c) Also \(n ^{\text {th }}\) term \(\boxed{T _{ n }= S _{ n }- S _{ n -1}}\) Note: (i) Sum of first n terms of an A.P. is of the form \(A n^{2}+B n\) i.e. a quadratic expression in n, in such case the common difference is twice the coefficient of \(n ^{2}\) . i.e. 2A (ii) \(n ^{\text {th }}\) term of an A.P. 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 ...
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