imperfectdark wrote:

I solved this (slightly) more algebraically in the sense that: -9 < x < 9 and -5<y<5 so multiplying the inequality of y by -2 we get

10 > -2y > -10, flipping the inequality for x around the same way we get 9 > x > -9 now adding them we obtain

19 > x - 2y > -19, and we see that the biggest prime number in this range is 17

why cant we solve as below (i know its wrong coz of the -2 multiplication)

-9<x<9

-10<2y<10

subtract

1<x-2y<-1

is it becoz the -2 in x-2y creates the problem while subtraction or we cannot subtract inequalities with same sign?? pls help

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