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Senior high school mathematics compulsory 5 exercises
Provide ideas and calculate for yourself.

Take (1) as an example.

Let z=a(7x-5y)+b(x+7y)

There is 7a+b=4

7b-5a=3

Find a and b and solve them with inequality knowledge.

Another way of thinking:

Draw a picture, draw the range of x and y (surrounded by three lines), then draw the objective function, and then calculate the intersection and vertex values.