cos(-25π/12)
= cos(25π/12)
= cos(2π+π/12)
= cos(π/12)
= cos{(π/6)/2}
= √{[1+cos(π/6)]/2}
= √{[1+√3/2]/2}
= √(2+√3)/2
= (√3+1)/(2√2)
= (√6+√2)/4