Question

Find $a,b$ such that $18,a,b,-3$ are in $AP$

Answer 1

Formula,

$a_n=a+(n-1)d$

Given,

$a_4=18+(4-1)d$

$-3=18+3d$

$\therefore d=-7$

$a=18+d=18-7=11$

$b=a+d=11-7=4$