maths~~~

Sol

設 z=ax+by

5=a*1+b*1，3=a*1+b* 2

a+b=5，a+2b=3

b=－2，a=7

z=7x－2y

題目改為另一部份隨y反變

z=ax+b/y

5=a*1+b/1，3=a*1+b/ 2

a+b=5，2a+b= 6

a=1，b=4

z=x+4/y

選(C)

subsititute x=1, y=1
only A and C gives the z value as 5

set x=1, y=2 for A and C
C gives z=3

So C is the answer