原式=1/2∫sin(2t)cos(2t)d(2t)=1/2∫sin(2t)d(sin(2t))=sin^2(2t)/4+C
解：(cos2t/sint)'=[(cos²t-sin²t)/sint]'=[cost·cot(t) -sint]'=(cost)'·cot(t)+cost·[cot(t)]' -(sint)'=-sint·(cost/sint) +cost·(-csc²t) -cost =-2cost -csc²t·cost =-(csc²t+2)cost ...
cosx-sinx =√2/2cosx-√2/2sinx =√2(1/2cosx-1/2sinx)=√2(cosπ/4cosx-sinπ/4sinx)=√2cos(x+π/4)和角公式：sin ( α ± β ) = sinα · cosβ ± cosα · sinβ sin ( α + β + γ ) = sinα · cosβ · cosγ + cosα · sinβ · cosγ + cos...
f'(t) = [ -2sin(2t).sint - cos2t. cost ] /(sint)^2