x∈(0, π/2);
cosx • sin( π/2 - x) - sinx • cos(π/2 - x) = 1/2
sin( π/2 - x)=cosx si cos(π/2 - x)=sinx
cosx • sin( π/2 - x) - sinx • cos(π/2 - x)=cosx•cosx-sinx•sinx
cos²x-sin²x=1/2
sin²x=1-cos²x
=> cos²x-1+cos²x=1/2
2cos²x=1/2+1
2cos²x=3/2
cos²x=3/4; x∈(0, π/2);
=> cosx=√(3/4)= √3 /2
=> x= π/6
