Question

If 7th and 13th term of an AP be 34 & 64 respectively, then it's 18th term is

a. 87
b. 88
c. 89
d. 90
Others : ​

Answer 1

Answer:

a+6d=34

a+12d=64

solving these two

d=5

a=4

for 18th term

=a+17d

=4+17×5

=4+85

=89

Answer 2

Answer:

a7= a+(n-1)d .

34=a+(7-1)d.

34=a+6d. similarly a13 is a+12d=64.

so by the estimation method: -->

34=a+6d ................a.

64=a+12d

____________

-30= -6d

d= 5.

by putting this in equation a . by which the value of a is :-->

34=a+6(5)

34=a+30.

34-30=a.

4=a.

so ,the 18th term is a+(n-1)d .

a18=4+(18-1)5.

a18=4+(17×5).

a18=4+85.

a18=89.

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