Question

What is the common difference of an A.P. in which a

21  a

7

= 84 ?

Answer 1

Solution:-

here, given a21 - a7 = 84 .................. ( 1 )

let the first term of the AP be a and the common difference be d.

an = a+ ( n-1) d

a21 = a+ ( 21- 1) d = a + 20d

a7 = a + ( 7 - 1) d= a+ 6d

Substituting the values in equation ( 1 )

a + 20d - ( a + 6d ) = 84

14d = 84

therefore, d= 6 (ans) .

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