3.n(n thuộc N)

Ư(30)=(30;1;2;3;5;6;10;15)

a)\(A=\left\{x\in N\left|x⋮3\right|x>3\right\}\)

b)\(B=\left\{x\in N\left|30⋮x\right|x\le30\right\}\)

\(x^n\left(x+1\right)-x^n-x^{n-1}=0\)

<=> \(x^{n+1}-x^{n-1}=0\)

<=> \(x^{n-1}\left(x^2-1\right)=0\)

<=> x = 0 ; x = 1; x = -1

37 x 13 - \(\frac{13}{24}\) + 37 x 12

=  37 x ( 13 + 12) - \(\frac{13}{24}\)

=  37 x 25 - \(\frac{13}{24}\)

=      925 - \(\frac{13}{24}\)

=       \(\frac{22187}{24}\)

\(37\times13-\frac{13}{24}+37\times12\)

\(=37\times13+37\times12-\frac{13}{24}\)

\(=37\times\left(13+12\right)-\frac{13}{24}\)

\(=37\times15-\frac{13}{24}\)

\(=505-\frac{13}{24}\)

\(=\frac{505\times24-13}{24}\)

\(=\frac{12107}{24}\)

\(A=\frac{11}{12}+13+\frac{12}{13}+14+\frac{13}{14}+12\)

\(A=1-\frac{1}{12}+13+1-\frac{1}{13}+14+1-\frac{1}{14}+12\)

\(A=3-\frac{1}{12}+13+\frac{1}{13}+14+\frac{1}{14}+12\)

\(A=\frac{1}{12}+12+\frac{1}{13}+12+\frac{1}{14}+12\)

\(A=\frac{143+182+164}{572}+36\)

\(A=36\frac{489}{572}\)

\(5!=1.2.3.4.5=120\)

\(4!-3!=1.2.3.4-1.2.3=24-6=18\)

\(\text{a)}5!=1.2.3.4.5=120\)

\(\text{b)}4!-3!=3!.\left(4-1\right)=1.2.3.3=18\)

558540 x=5

              y=0

B chia hết cho 2 và 5 nên y = 0 

B chia hết cho 9 

\(2x+8+5+4+0⋮9\) 

8 + 5 + 4 = 17 chia 9 dư 8 nên 2x chia 9 dư 1 \(\Rightarrow\) x chia 9 dư 5 

x = 5