Need help understanding a problem.

Question

In ∆RST, R is a right angle. If cos T =12/13, determine cos S.

Answer 1

T = 90° - S or S = 90° - T. cosS = cos(90° - T) = sinT = (1 - (12/13)²)^1/2 = 5/13

Answer 2

another solution using the Pythagorean Theorem...

cosT=12/13

cos=adjacent/hypotenuse

13^2=12^2+RS^2

169=144+RS^2

25=RS^2

RS=5

cosS=5/13