а) 3/√5 = (3√5)/(√5*√5) = (3√5)/5
б) 5/(2√3) = (5*2√3)/(2√3 * 2√3) = (10√3)/12 = (5√3)/6
в) 10/(√21-3) = 10(√21+3)/(√21-3)(√21+3) = 10(√21+3)/(21-9) = 5(√21+3)/6
