Question

If 1st 12 terms of an AP is -13 and the sum of 1st 4 terms of AP is 24. Find sum of 1st 20 terms

Answer 1

Answer:

Step-by-step explanation:

12/2[ 2a + 11d]= -13

6 [ 2a + 11d]= -13

12a + 66d = -13

4/2[ 2a + 3d]= 24

2[ 2a + 3d]= 24

2a + 3d= 12

solve

12a +66d=-13

12a + 18d= 72

-      -           -

_____________

       48d= - 85

            d= -85/48

2a + 3* -85/48= -13

2a = -13+85/12

a = -71/24

20/2[ 2*(-71/24)  + 19 * -85/12]

10[ -71/12-1615/12

10[-1686/12]

=1405