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CBSE Class 10 Answered

Find the sum of the first 17 term of an AP whose 4th term = -15 and 9th term = -30.
Asked by arindeep.singh | 26 Jun, 2020, 03:34: PM
answered-by-expert Expert Answer
4th term = -15 and 9th term = -90
a + 3d = -15 ... (i) and
a + 8d = -30 ... (ii)
Subtracting (ii) from (ii), we get
5d = -15
d = -3
Substituting the value of d in (i), we get
a + 3(-3) = -15
a - 9 = -15
a = -6
Sum of n terms if given by
S subscript n equals n over 2 open square brackets 2 a plus left parenthesis n minus 1 right parenthesis d close square brackets S subscript 17 equals 17 over 2 open square brackets 2 cross times left parenthesis negative 6 right parenthesis plus left parenthesis 17 minus 1 right parenthesis left parenthesis negative 3 right parenthesis close square brackets equals 17 over 2 open square brackets negative 12 minus 48 close square brackets equals 17 over 2 open square brackets negative 60 close square brackets equals 17 open parentheses negative 30 close parentheses equals negative 510
Answered by Renu Varma | 28 Jun, 2020, 05:14: PM

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