Question

find the sum of 21 terms of an ap whose middle term is 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: -

step by step explanation

Let first no be a

And common difference be d

S21 =?

middle term will T11 = 20

T11= a+(11-1)d

20= a+10d

a= 20-10d______(1)

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

= 21/2[2a +20d]

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

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

=21/2[40]

=420________answer