Question

Find the sum of 20 terms of an AP whose third term is 5 and the seventh term is 9

Answer 1

Answer:

Hy mate here is ur answer°↓↓

a³=a²+d

5=a²+d-->(1)

A7=A6+d

9=A5+2d-->(2)

°•° S20=N [2a+(n-1)d]

2

Subtract (1)&(2)

a+d=5

a+2d=9.

-d=-4

d=4-->(3)

now (3)in(1)

5=a²+4

a²=1(5-4)

a²=a+d

-1=a+4

a=5(4+1)

now,As we Know a=5 & d=4

S20=N [2a(n-1)d]

2

=20 [2×5(20-1)4]

2

=10[10×76]

=10×760

°•°S20=7600

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