Question

If the sum of the first 7 terms is 49 find the sum of its first 10 terms

Answer 1

Answer:

Step-by-step explanation:

a12 = -13

a + 11d = -13

a = -13 - 11d -------1

Sum of first 4 terms

a + a + d + a + 2d + a + 3d = 24

4a + 6d = 24

2a + 3d = 12

2 ( -13 -11d ) + 3d = 12

-26 -22d + 3d = 12

-19d = 38

d = -2

Putting in 1

a = -13 - 11d

a = -13 - 11(-2)

a = -13 +22

a = 9

Sum of 10 terms Sn=n/2(2a+(n-1)d) Sn=10/2(2(9)+(10-1)(-2)) S=5(18-18) S=0  

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