Question

Find the sum of first 21 terms of an ap whose middle term os 20​

Answer 1

For an ap ...first term =a and d be the common difference...S21=?

middle term i.e. t11 =20 .

therefore t11 =a+ (11-1)d

therefore 20=a + 10d....a=20-10d...(1)

S21=21/2[2a + (21-1)d]

=21/2[2a +20d]

=21/2[2(20-10d)+20d]...from(1)

=21/2[40-20d+20d]

=21/2×40

=21×20

S21=420

Answer 2

Answer:

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Step-by-step explanation:

ANSWER

Given,

middle term t

11

=20

t

n

=a+(n−1)d

t

11

=a+10d=20

⇒a=20−10d

S

n

=

2

n

[2a+(n−1)d]

S

21

=

2

21

[2(20−10d)+(21−1)d]

=

2

21

[40]

∴S

21

=420