a, Ta có: 30 = 2.3.5; 45 = 32.5

BCNN(30,45) = 2. 32.5 = 90

Suy ra:  x∈BC(30,45) = B(90) = {0;90;180;270;360;450;540;...}

Mà x < 500 => x∈{0;90;180;270;360;450}

b, Ta có: 34 = 2.17; 85 = 5.17

BCNN(34;85) = 2.17.5 = 170

Suy ra: x∈BC(34,85) = B(170) = {0;170;340;510;680;850;1020;...}

Mà 500 < x < 1000 => x∈{510;680;850}

c, Ta có: 12 = 22.3; 21 = 3.7; 28 = 22.7

BCNN(12,21,28) =  22.3.7 = 84

Suy ra: x∈BC(12,21,28) = B(84) = {0,84,168,252,336,...}

Mà 150 < x < 300 => x∈{168;252}

d, Ta có: 65 = 5.13; 45 = 32.5; 105 = 3.5.7

BCNN(65;45;105) = 32.5.7.13 = 4095

Suy ra: x∈BC(65,45,105) = B(4095) = {0;4095;8190;12285;...}

Mà x có bốn chữ số => x∈{4095;8190}

e, Vì x⋮39; x⋮65, x⋮91 nên x∈BC(39,65,91)

Ta có: 39 = 3.13; 65 = 5.13; 91 = 7.13

BCNN(39,65,91) = 3.5.7.13 = 1365

Suy ra x∈BC(39,65,91) = B(1365) = {0,1365,2730,...}

Mà 400 < x < 2600 => x = 1365