Question

The ratio of 11 term to 18 term of an ap is 2:8 find the ratio of 5 term to 21 term and also the ratio of sum of first five term to the sum of first 21 term

Answer 1

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Answer 2

Given: 11th term is 2,and 18th term is 8

a11=a+10d

a+10d=2. ........eq(1)

similarly,

a18=a+17d

a+17d=8. ...........eq(2)

by eliminating both equation(1)&(2),we get

a+10d=2

a+17d=8

- - -

-7d=-6

d=6/7

d=0.8

a=-6

putting this value

a5=a+4d

=-6+4(0.8)

=-6+3.2

=-2.8

a21=a+20d

=-6+20(0.8)

=-6+16

=10

So,the ratio is -2.8:10