Question

7.If 18, a, b, - 3 are in A.P., then a+b = ?

(1 Point)

1212

1515

1616

1111

please give the answer as fast as you can

please ​

Answer 1

Answer: 15

Explanation:

General terms in AP

f, f+d, f+2d, f+3d, .... , where f= first term and d= common difference.

The given A.P. : 18, a, b, - 3

here, f= 18

`f+d= a ...(i)\\\\f+2d = b ...(ii)\\\\f+3d= -3 ...(iii)\\\\`

Put f= 18 in (iii) ,

`18+3d=-3\\\\\Rightarrow\ 3d= -3-18\\\\\Rightarrow\ 3d= -21\\\\\Rightarrow\ d=-7`

Put f= 18 and d= -7 in (i) and (ii) , we get

`a=18+(-7)=11\\\\b=18+2(-7)\\\\\Rightarrow\ b=18-14\\\\\Rightarrow\ b=4`

Now,
`a+b= 11+4=15`

Hence, the correct answer is "15".