Answer
4th year—23 cranes born
46 cranes in the first four years
Work Step by Step
$a_n=n^2+2n-1$
4th year
$n=4$
$a_n=n^2+2n-1$
$a_4=4^2+2*4-1$
$a_4 =16+8-1$
$a_4 = 23$
4th year—23 cranes born
First 4 years
$n=1$
$a_n=n^2+2n-1$
$a_1=1^2+2*1-1$
$a_1=1+2-1$
$a_1=2$
$n=2$
$a_n=n^2+2n-1$
$a_2=2^2+2*2-1$
$a_2=4+4-1$
$a_2=7$
$n=3$
$a_n=n^2+2n-1$
$a_3=3^2+2*3-1$
$a_3 =9+6-1$
$a_3=14$
$2+7+14+23$
$46$
46 cranes in the first four years