7/2 (2a+6d) = 13/2 (2a+12d)
7a+21d = 13a+78d
6a + 57d = 0
2a + 19d = 0
S20 = 20/2 (2a+19d) = 0
The sum of first 7 terms of an ap is equal to the sum of its first 13 terms . find the sum of its ist 20 terms
2 answers
Use the formula for the sum of terms
(7/2)(2a + 6d) = (13/2)(2a + 12d)
times 2, then expand
14a + 42d = 26a + 156d
-12a = 114d
a = -19/2 d
sum(20) = 10(2a + 19d)
= 20a + 190d
= 20(-19/2 d) + 190d
= 0
(7/2)(2a + 6d) = (13/2)(2a + 12d)
times 2, then expand
14a + 42d = 26a + 156d
-12a = 114d
a = -19/2 d
sum(20) = 10(2a + 19d)
= 20a + 190d
= 20(-19/2 d) + 190d
= 0