Question

Ma ajuta cineva sa rezolv?

Answer 1

daca x∈ ( 0 , π/2)  sinx , cosx > 0
sin² a +cos²a =1
( 5/13)² +cos²a =1
cos²a = 1 - 25 /169 
cos²a =  ( 169 -25) /169
cos²a = 144 /169 
cosa = √144 /√169 = 12 /13

Answer 2

$Observam~ca~cos~a \geq 0.\\ sin ^{2} a+cos ^{2} a=1=\ \textgreater \ cos~a= \sqrt{1- sin^{2} a} = \sqrt{1 -\frac{25}{169} }= \sqrt{\frac{144}{169} }= \frac{12}{13} .$