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CBSE Class 10 Answered

If the sum of first 10 terms of an A.P. is 140 and that of 16 terms is 320;find the sum of first n terms
Asked by singh.ayushkumar4343 | 11 Feb, 2018, 09:06: PM
answered-by-expert Expert Answer
Sum of first 10 terms = 140
S10=5(2a+9d)
28=2a+9d -----(1)
 
S16 = 8(2a+15d)
320= 8(2a+15d)
40=2a+15d -----(2)
from (1) and (2), performing (2)-(1),
 
12=6d
d=2.
Find a.
 
Put d=2 in (1),
28=2a+18
28-18=2a
10=2a
a=5.
 
Sn = n/2 (2a + (n-1)d)
Sn = n/2(10+(n-1)2)
Expression for sum of nth terms.
 
Answered by | 11 Feb, 2018, 10:22: PM

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