sin²α+cos²α=1
ex 3: sin²(11π/12) + cos²(11π/12) =1
[ (√6-√2)/4]² + cos²(11π/12) = 1
(√6-√2)²/4² + cos²(11π/12) = 1
((√6)²+(√2)²-2×√6×√2))/16 + cos²(11π/12) =1
(6+2-2√12)/16 + cos²(11π/12) = 1
(8-2√12)/16 + cos²(11π/12) = 1
cos²(11π/12)=1- (8-2√12)/16
cos²(11π/12)=16/16 - (8-2√12)/16
cos²(11π/12) =[16-(8-2√12)]/16
cos²(11π/12)= (16-8+2√12)/16
cos²(11π/12)=(8+2√12)/16
cos²(11π/12) =(√6+√2)²/4²
cos²(11π12)= [(√6+√2)/4]²
cos(11π/12) = (√6+√2)/4