# Arithmetic progression

• Apr 11th 2010, 04:33 PM
sigma1
Arithmetic progression
the$\displaystyle N^{th}$ term of an A.P is denoted by $\displaystyle U_{n}$ and the sum of the $\displaystyle 1^{st}n$ terms by $\displaystyle S_{N}$ in a certain A.P ;; $\displaystyle U_{5}+U_{16}=44$ and$\displaystyle S_{18}=3S_{10}.$calculate the value of the first term and the common difference.

i have been trying to get two equations from the given equation using the rules for arithmetic progression but i have been unable to do so..

i really need some help coming up with these equations.
• Apr 11th 2010, 06:25 PM
tonio
Quote:

Originally Posted by sigma1
the$\displaystyle N^{th}$ term of an A.P is denoted by $\displaystyle U_{n}$ and the sum of the $\displaystyle 1^{st}n$ terms by $\displaystyle S_{N}$ in a certain A.P ;; $\displaystyle U_{5}+U_{16}=44$ and$\displaystyle S_{18}=3S_{10}.$calculate the value of the first term and the common difference.

i have been trying to get two equations from the given equation using the rules for arithmetic progression but i have been unable to do so..

i really need some help coming up with these equations.

Using the formula or the n-th term in an A.P., for all $\displaystyle n\,,\,\,u_n=u_1+(n-1)d\Longrightarrow 44=u_5+u_{16}=$ $\displaystyle u_1+4d+u_1+15d=2u_1+19d\Longrightarrow (I)\,\,2u_1+19d=44$ , and using the formula for the sum of consecutive elements in an A.P. we get

$\displaystyle 3S_{10}=3\cdot \frac{10}{2}\left(2u_1+9d\right)=\frac{18}{2}\left (2u_1+17d\right)=S_{18}$ $\displaystyle \Longrightarrow 10u_1+45d=6u_1+51d\Longrightarrow (II)\,\,2u_1-3d=0$ , and there you have two linear eq's in two unknowns: $\displaystyle u_1\,\,\,and\,\,\,d$

Tonio