Câu 1:
Gọi số đó là x.Ta có:
x chia 5 dư 1 =>(x-1) chia hết cho 5 =>(x-1)+10 chia hết 5=>(x+9)chia hết cho 5
x chia 7 dư 5 =>(x-5) chia hết cho 7 =>(x-5)+14 chia hết 7=>(x+9)chia hết cho 7
=> x+ 9 ∈ BCNN(5;7)
Ta có:
5=5.1
7=7.1
=>BCNN(5, 7)=5.7=35
Mà x+9=35
=>x=35-9
=>x=26
Câu 2:
a)(-213).68+32(-213)
=(-213)(68+32)
=-213.100
=-21 300
b)23và1/3:(-5/7)-13và1/3:(-5/7)
=70/3:(-5/7)-40/3:(-5/7)
=70/3x-7/5-40/3x-7/5
=-7/5(70/3-40/3)
=-7/5x10
=-14
c)5.2^30.3^18-2^29.3^20/5.2^28.3^19-7.2^29.3^18
=5.2^28.2.3^18-2^28.2.3^18.3^2/5.2^28.3^18.3-7.2^28.2y.3^18.3^2
=5.2^28.2^2.3^18-2^28.2.3^18.3^2/5.2^28.3^18.3-7.2^28.3^18.3^2
=2^28.3^18(5.2^2)-2^28.3^18(2.3^2)/2^28.3^18(5.3)-2^28.3^18(7.3^2)
=2^28.3^18.20-2^28.3^18.18/2^28.3^18.15-2^28.3^18.63
=2^28.3^18(20-18)/2^28.3^18(15-63)
=2^28.3^18.2/2^28.3^18.-48
=2^28.3^18/2^28.3^18.-24
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