sin70°×sin50°×sin30°×sin10°
=cos20°×cos40°×(1/2)×sin10°
=(1/2)×sin10°×cos20°×cos40°
=(1/2)×sin10°×cos10°×cos20°×cos40°÷cos10°
=(1/4)×2sin10°cos10°×cos20°×cos40°÷cos10°
=(1/4)×sin20°×cos20°×cos40°÷cos10°
=(1/8)×2sin20°cos20°×cos40°÷cos10°
=(1/8)×sin40°×cos40°÷cos10°
=(1/16)×2sin40°cos40°÷cos10°
=(1/16)×sin80°÷cos10°
=(1/16)×cos10°÷cos10°
=1/16