Mechanics Question

[KeepGoingForIt]

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A box of mass m is sitting on an incline of 45 degrees and requires an applied force F up the incline to get the box to begin to move. What is the max coefficient of static friction?

Answer: [(square root of 2 )(F)/mg] -1

Why is it minus 1? I am totally lost...

 

[KeepGoingForIt]

Ahh I figured it out. Math. Too much math. Hope it's not on the test!