А)
[ 12x^3 + 3x + 1 ] / [ 6x^3 + x^2 + x - 2 ] = [ 12 + 3/x^2 + 1/x^3 ] / [ 6 + 1/x + 1/x^2 - 2/x^3 ] -> 12/6 = 2
б)
[ √(x^2 - 7) - 3 ] / [ x^2 - 4x ] = [ x^2 - 7 - 9 ] / [ (x^2 - 4x)*(√(x^2-7)+3) ] = [ (x-4)(x+4) ] / [ x(x-4)(√(x^2-7)+3) ] = [ x + 4 ] / [ x*(√(x^2-7)+3) ] -> [4+4] / [4*(√(16-7)+3)] = 8 / (4 * 6) = 1/3
в)
4x^2 / (1 - cos4x) = 4x^2 / (1 - cos^2(2x) + sin^2(2x)) = 4x^2 / (2sin^2(2x)) = 2x^2 / sin^2(2x) ~ {sin2x -> 2x} ~ 2x^2 / (2x)^2 -> 1/2