Answer :
Answer: 15
Step-by-step 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
[tex]f+d= a ...(i)\\\\f+2d = b ...(ii)\\\\f+3d= -3 ...(iii)\\\\[/tex]
Put f= 18 in (iii) ,
[tex]18+3d=-3\\\\\Rightarrow\ 3d= -3-18\\\\\Rightarrow\ 3d= -21\\\\\Rightarrow\ d=-7[/tex]
Put f= 18 and d= -7 in (i) and (ii) , we get
[tex]a=18+(-7)=11\\\\b=18+2(-7)\\\\\Rightarrow\ b=18-14\\\\\Rightarrow\ b=4[/tex]
Now, [tex]a+b= 11+4=15[/tex]
Hence, the correct answer is "15".