Lời giải  
x=0,y=0x equals 0 comma y equals 0
𝑥=0,𝑦=0
hoặc x=9,y=0x equals 9 comma y equals 0
𝑥=9,𝑦=0
hoặc x=4,y=5x equals 4 comma y equals 5
𝑥=4,𝑦=5
.
72*∶*=972 * colon * equals 9
72*∶*=9
; 408*∶*=6408 * colon * equals 6
408*∶*=6
; 31*31∶*=131 * 31 colon * equals 1
31*31∶*=1
; 8*215∶*=28 * 215 colon * equals 2
8*215∶*=2
; 3*12∶*=33 * 12 colon * equals 3
3*12∶*=3
.
5*3125 * 312
5*312
chia 99
9
dư 4∶*=44 colon * equals 4
4∶*=4
.
41*∶*=1,4,741 * colon * equals 1 comma 4 comma 7
41*∶*=1,4,7
; 510510
510
: luôn chia hết cho 33
3
và không chia hết cho 99
9
; 61*16∶*=1,4,761 * 16 colon * equals 1 comma 4 comma 7
61*16∶*=1,4,7
; 7*123∶*=2,5,87 * 123 colon * equals 2 comma 5 comma 8
7*123∶*=2,5,8
; 3*4∶*=2,5,83 * 4 colon * equals 2 comma 5 comma 8
3*4∶*=2,5,8
.
25*3*∶*=025 * 3 * colon * equals 0
25*3*∶*=0
(chữ số cuối), *=2,5,8* equals 2 comma 5 comma 8
*=2,5,8
(chữ số giữa).
a=7,b=2a equals 7 comma b equals 2
𝑎=7,𝑏=2
.
A=123xcap A equals 123 x
𝐴=123𝑥
: chẵn khi x∈{0,2,4,6,8}x is an element of start-set 0 comma 2 comma 4 comma 6 comma 8 end-set
𝑥∈{0,2,4,6,8}
; lẻ khi x∈{1,3,5,7,9}x is an element of start-set 1 comma 3 comma 5 comma 7 comma 9 end-set
𝑥∈{1,3,5,7,9}
; chia hết cho 33
3
khi x∈{0,3,6,9}x is an element of start-set 0 comma 3 comma 6 comma 9 end-set
𝑥∈{0,3,6,9}
; chia hết cho 55
5
khi x∈{0,5}x is an element of start-set 0 comma 5 end-set
𝑥∈{0,5}
; chia hết cho 99
9
khi x=0x equals 0
𝑥=0
hoặc x=9x equals 9
𝑥=9
.

Lời giải  
x=0,y=0x equals 0 comma y equals 0
𝑥=0,𝑦=0
hoặc x=9,y=0x equals 9 comma y equals 0
𝑥=9,𝑦=0
hoặc x=4,y=5x equals 4 comma y equals 5
𝑥=4,𝑦=5
.
72*∶*=972 * colon * equals 9
72*∶*=9
; 408*∶*=6408 * colon * equals 6
408*∶*=6
; 31*31∶*=131 * 31 colon * equals 1
31*31∶*=1
; 8*215∶*=28 * 215 colon * equals 2
8*215∶*=2
; 3*12∶*=33 * 12 colon * equals 3
3*12∶*=3
.
5*3125 * 312
5*312
chia 99
9
dư 4∶*=44 colon * equals 4
4∶*=4
.
41*∶*=1,4,741 * colon * equals 1 comma 4 comma 7
41*∶*=1,4,7
; 510510
510
: luôn chia hết cho 33
3
và không chia hết cho 99
9
; 61*16∶*=1,4,761 * 16 colon * equals 1 comma 4 comma 7
61*16∶*=1,4,7
; 7*123∶*=2,5,87 * 123 colon * equals 2 comma 5 comma 8
7*123∶*=2,5,8
; 3*4∶*=2,5,83 * 4 colon * equals 2 comma 5 comma 8
3*4∶*=2,5,8
.
25*3*∶*=025 * 3 * colon * equals 0
25*3*∶*=0
(chữ số cuối), *=2,5,8* equals 2 comma 5 comma 8
*=2,5,8
(chữ số giữa).
a=7,b=2a equals 7 comma b equals 2
𝑎=7,𝑏=2
.
A=123xcap A equals 123 x
𝐴=123𝑥
: chẵn khi x∈{0,2,4,6,8}x is an element of start-set 0 comma 2 comma 4 comma 6 comma 8 end-set
𝑥∈{0,2,4,6,8}
; lẻ khi x∈{1,3,5,7,9}x is an element of start-set 1 comma 3 comma 5 comma 7 comma 9 end-set
𝑥∈{1,3,5,7,9}
; chia hết cho 33
3
khi x∈{0,3,6,9}x is an element of start-set 0 comma 3 comma 6 comma 9 end-set
𝑥∈{0,3,6,9}
; chia hết cho 55
5
khi x∈{0,5}x is an element of start-set 0 comma 5 end-set
𝑥∈{0,5}
; chia hết cho 99
9
khi x=0x equals 0
𝑥=0
hoặc x=9x equals 9
𝑥=9