Question

Answer 1

a: \(12=2^2\cdot3;9=3^2\)

=>\(BCNN\left(12;9\right)=3^2\cdot2^2=36\)

\(x⋮9;x⋮12\)

=>\(x\in BC\left(9;12\right)\)

mà x nhỏ nhất

nên x=BCNN(9;12)

=>x=36

b: \(9=3^2;15=3\cdot5\)

=>\(BCNN\left(9;15\right)=3^2\cdot5=45\)

\(x⋮9;x⋮15\)

=>\(x\in BC\left(9;15\right)\)

=>\(x\in B\left(45\right)\)

=>\(x\in\left\{45;90;135;180;...\right\}\)

mà 60<=x<140

nên \(x\in\left\{90;135\right\}\)

c:

\(21=3\cdot7;28=2^2\cdot7\)

=>\(BCNN\left(21;28\right)=2^2\cdot3\cdot7=84\)

 \(x⋮21;x⋮28\)

=>\(x\in BC\left(21;28\right)\)

=>\(x\in B\left(84\right)\)

=>\(x\in\left\{84;168;252;336;420;...\right\}\)

mà 100<x<=290

nên \(x\in\left\{168;252\right\}\)