sin2atana
=2sinacosa*sina/cosa
=2sin²a
√(1-cosx)=√[2sin²(x/2)]=√2|sin(x/2)|
sin π/10×cos π/5
=cos(π/2-π/10)cosπ/5
=cosπ/5cos2π/5
=(2sinπ/5*cosπ/5cos2π/5)/(2sinπ/5)
=(2sin2π/5*cos2π/5)/(4sinπ/5)
=(sin4π/5)/(4sinπ/5)
=sinπ/5/(4sinπ/5)
=1/4