a, xy -x +2y=3
⇒x(y-1)=3-2y
⇒x=$\frac{3-2y}{y-1}$
⇒x=$\frac{-2(y-1)+1}{y-1}$
⇒x=-2+$\frac{1}{y-1}$
y-1∈U(1)={±1}
y-1=1⇒y=2⇒x=-1 (thoa man)
y-1=-1⇒y=0⇒x=-3 (thoa man)
Vay (x,y)∈{(-1;2);(-3;0)}
b, Neu P=2 ⇒p+6=8 (loai)
Neu P=3⇒p+6=9 (loai)
Meus P=5⇒p+6=11; p+8=13; p+12=17; p+14=19 (thoa man)
Neu p>5 maf p laf so nguye to ⇒p co 1 trong 4 dang sau: 5k+1; 5k+2; 5k+3; 5k+4
Voi p=5k+1⇒p+14=5k+14 (loai)
Voi p=5k+1⇒p+8=5k+9 (loai)
Voi p=5k+2⇒p+12=5k+14 (loai)
Voi p=5k+3⇒p+6=5k+9 (loai)
Vay p=5
