Question

The 10th and 15th term of an a.p are -5 and -15/2 respectively. The sum of the first 20th term is

Answer 1

Answer:

-17/4

Step-by-step explanation:

a10=-5

or a + 9d=-5 ___(1)

a15=-15/2

or a + 14d=-15/2 ___(2)

On subtracting eq. (1) from (2),we get

5d=15/2

d=3/2

Now, putting d=3/2 in eq. (1),we get

a = -37/2

Now, a20=a +19d=-37/2+19(3/2)

=10

Sum of the first 20th term=1/2(a+a20)

=1/2(-37/2+10)=-17/4 Ans.

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