How to prove a negative number?

Question

Hi, i need some help proving that "c" in this equation is a minus number.

a + b -1=c x (d^2)/(e^2) x (f^2) , so ( a plus b minus 1 = c times d squared divided by e squared times f squared).

Answer 1

a + b -1 = cd² / (e²f²)

d², e² & f² are all positive, since they are squares, so the sign of the right side is determined by c

if a+b>1 then the left side is positive so c must be positive

if a+b<1 then the left side is negative so c must be negative

so c is only negative when a+b is less than 1

,..,

Answer 2

do indirect proof for this one