单选题
sin163°sin223°+sin253°sin313°=______。
A．
B．
C．
D．
A．
B． C． D．
【正确答案】
B
【答案解析】[解析] sin163°sin223°+sin253°sin313°
=sin(180°-17°)sin(180°+43°)+sin(270°-17°)sin(360°-43°)
=-sin17°sin43°+cos17°cos43°
=cos(17°+43°)=cos60°
=
=sin(180°-17°)sin(180°+43°)+sin(270°-17°)sin(360°-43°)
=-sin17°sin43°+cos17°cos43°
=cos(17°+43°)=cos60°
=