Tìm x

- Hướng dẫn giải

a, Ta có: x$\in$B(5) = {0;5;10;15;20;25;...}

Mà x < 20 => x$\in${0;5;10;15}

b, Ta có: x$⋮$24; x$⋮$45 => x$\in$BC{24;45}

24 = $2^3.3$; 45 = $3^2.5$ => BCNN(24;45) = $2^3.3^2.5$ = 360

=> BC(24;45) = B(360) = {0;360;720;...}

Mà 200 < x < 500 => x = 360

c, Ta có:

452 = px+32 => 420$⋮$x

321 = qx+21 => 320$⋮$x

=> x$\in$ƯC(420;320)

420 = $2^2.3.5.7$

320 = $2^6.5$

=> ƯCLN(420;320) = 20 = $2^2.5$

=> ƯC(420;320) = Ư(20) = {1;2;4;5;10;20}

Vậy x$\in${1;2;4;5;10;20}