cos(x+30°)=5/13,x为锐角,
所以sin(x+30°) = 12/13
cosx = cos[(x+30) - 30]
= cos(x+30)cos30 + sin(x+30)sin30
= 5/13*√3/2 + 12/13*1/2
= (5√3 +12)/26