Sum of 1st 15 terms is 105. Sum of next 15 terms is 780. Find 1st 3 rerms
Answers
Answered by
1
given sum of 1st 15 terms is 105
so,
Sn=n/2[2a+(n-1)d]
105=15/2[2a+(15-1)d]
105=15/2[2a+14d]
105=15[a+7d]
105=15a+105d---------(1)
given sum of next 15 terms is 780 and 1st terms is 105
885=15[2a+29d]
885=30a+435 ------------(2)
so from 1 and 2 we get a and d
so,
Sn=n/2[2a+(n-1)d]
105=15/2[2a+(15-1)d]
105=15/2[2a+14d]
105=15[a+7d]
105=15a+105d---------(1)
given sum of next 15 terms is 780 and 1st terms is 105
885=15[2a+29d]
885=30a+435 ------------(2)
so from 1 and 2 we get a and d
Similar questions