Find out the next number in the following Series
5 , 7 , 12 , 13 , 9 , ? .
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Let's name the numbers
$x_1, x_2, x_3, x_4, x_5, x_6$.
We need to find $x_6$.
Adding the numbers 2 by 2 in such a a way that their indexes sum up to 7 we get:
$x_3 + x_4 = 12+13 = 25 = 5^2$
$x_2 + x_5 = 7+9 = 16 = 4^2$
$x_1 + x_6 = 5+4 = 9 = 3^2$
It's about perfect squares