The 2nd and 3rd are indeed false. Looks like the minimums for both are 0 at (0, 0). (I didn't get a chance to look at the 1st.)
Can you guys check me on this, and if I'm not right help me out lol??
Minimize C=5x+4y {3x+4y>=32, x+4y>=2, x<=12, y<=15} x>=0 and y>= 0?
Solution: The minimum is 32 at (0,8)
True
Minimize C=4x+5y {2x-y<=0, 0<=y<=10, 0<=x<=10} x>=0, y>=0?
Solution: The minimum is 70 at (5,10)
False
Minimize C=7x+2y {x+3y<=108, 7x+4y<=280} x>=0, y>=0?
Solution: The minimum is 280 at (40,0)
False