Question

If 21, a, b and -3 are in A.P., then find the value of (a + b).
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Answer 1

Answer:

$Value \: of \: (a+b) \: is \: 18.$

Explanation:

a4 = a + (n-1)d

-3 = 21 + (4-1)d

-3-21 = 3d

-24 = 3d

d = -8

Putting the value of d in,

a2 = 21+ (1)-8

= 21-8

= 13

a3= 21 + (2)-8

= 21-16

= 5

a = 13 and b = 5

So, the value of (a+b) is 18.