Question

Chapter name Arithmatic progreesion .
Question no1; In an A.P., THE SUM OF N TERMS,COMMON DIFFERENCE AND LAST
term are 136,4 and 31respectively. find the value of n.

QUESTION NO 2;;H ow many term of the series 5+7+9+....must be taken in order that the sum may be 480?

QUESTION NO3;;find the three numbers in A.P such that their sum is 12 and the sum of their cubes is 408 .

Answer 1

ans 2  5+7+9 ....
sn = 480  
a =5 
d=2 
sn= n/2 [2a +(  n-1 ) d ]
480 = n/2 [ 2(5) + (n-1) 2 ]
480  = n/2 [10 +2n - 2] 
480 = n/2 [8+2n]
480 = 4n + n² 
n² +4n -480
apply d formula   -b +/- √b²-4ac/2a
you get n as 20  or -24 
ans is 20