Question 976580
There is an identity for Tangent of the difference of 2 angles - 

{{{tan(A-B)= (tanA-tanB)/(1+tanAtanB)}}}

here, A = 45, B = x, let's substitute :

{{{tan(45-x)= (tan45-tanx)/(1+tan45tanx)}}}

and one of the tan values I know by heart is tan 45 = 1. I hope you know this. 

{{{tan(45-x)= (1-tanx)/(1+tanx)}}}