Question

find the sum of first 21st terms in a. p. whose 1st term is 7 and 3rd term is 25​

Answer 1

Answer:

Step-by-step explanation:

By formula :- an = a+(n-1)d ,

a3 = a+(3-1)d

a3 = 7+2d

25 = 7+2d

2d = 25-7

2d = 18

••••••••••••••d = 9••••••••••••••••

So, as d=9 therefore,

An = a+ (n-1)d

A21 = 7+(21-1)d

A21 = 7 + (20)d

A21 = 7 + (20)9

A21 = 7 + 180

A21 = 187

•••••••••••••••A21 = 187••••••••••••••••

•°• Sn = n/2 (a+An)

Sn = 21/2 (7+ 187)

Sn = 21/2 (194)

Sn = 21(97)

Sn = 2037

••••••S21 = 2037•••••••••••••

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