bc is cool, but it can not access shell variables directly, but you can pass them with <<<

also there is no „pi“ constant in bc that you could use in interactive mode.

let one assume you would like to calculate extend „umfang“ of a circle with radius 5

the formula

Extend/Umfang U = 2 · π · r

the bash

pi=$(echo "scale=10; 4*a(1)" | bc -l); # calc value of pi 10 digits and store it in variable $pi
echo $pi; # test output

bc <<< "scale=10 ; 2*$pi*5"; # do the calculation