知不知道這兩條數學題甚樣做?(星期一要交)

1.
A.1533: 1+5+3+3 = 12
B.1812: 1+8+1+2 = 12
C.1417: 1+4+1+7 = 13
D.1902: 1+9+0+2 = 12
∴答案為C.
2.
A+B=8 ...(1)
AB+BA=±18 ...(2)
From (1): B=8-A ...(3)
put (3) into (2),
A(8-A)+(8-A)A=±18
8A-A^2 + 8A-A^2 = ±18
Case 1:
2A^2 - 16A + 18 = 0
∴No Solution.
Case 2:
2A^2 - 16A - 18 = 0
A=9 or A=-1
Case 2(i):
sub A=9 into (1):
9+B = 8
B=-1
Sub A=-1 into (1):
-1+B=8
B=9
∴A=9,B=-1 or A=-1,B=9

1.C.1417(14+1+7=22)
(&A.15+3+3=21
&B.18+1+2=21
&D.19+0+2=21)



(2)是
A B
+B A
_____
1 8
嗎?

answer:我唔知.....

1. C 因為其他的四個數相加後will become12, only this one is 13.