the 5th and 15th term of an AP are 13 and 17 respectively. find the sum of the first 21 terms of the AP
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Answer:
Solution is 323.4.
Step-by-step explanation:
a+(5-1)d = a+4d =13
a+(15-1)d= a+14d = 17
4d-14d= 13-17
=> -10d= -4
=> d= 0.4
a+4*0.4=13
=>a= 13-1.6= 11.4
sum of 21 terms
= 21/2 { 2*11.4+(21-1)*0.4)
= 21(11.4+4)
=21*15.4
=323.4
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