Question

Find the sum of first 20 terms of an AP whose 4th term is 18 and 9th term is 38

Answer 1

Answer:

81

Step-by-step explanation:

Do the simaltanious equation and fine the value of d and a

then put the values of obth and fine the 20th term

Answer 2

880 is the sum of first 20 terms of an AP whose 4th term is 18 and 9th term is 38.

Step-by-step explanation:

given :

$a_{4}=18$

$18=a+(4-1)d$

$18=a+3d$...[1]

$a_9=38$

$38=a+(9-1)d$

$38=a+8d$..[2]

[1] - [2]

$18-38=a+3d-(a+8d)$

$-20=-5d$

$d=\frac{-20}{-5}=4$

$18=a+(4-1)\times 4$

$a=18-12=6$

Sum of n term is given by:

$S_{n}=\frac{n}{2}\times (2a+(n-1)d)$

n = 20 , a = 6 , d = 4

$S_{20}=\frac{20}{2}\times (2\times 6+(20-1)4=880$

880 is the sum of first 20 terms of an AP whose 4th term is 18 and 9th term is 38.

Learn more about : Arithmetic progression

https://brainly.com/question/12172540

https://brainly.in/question/3360522

#learnwithbrainly