sin315°*sin(-1260°)+cos570°*sin(-840°)
=-sin45*(-sin180)+cos150(-sin120)
= sin45*sin180-cos150*sin120
=（根号2）/2*1-（-根号3）/2*（根号3）/2
=（根号2）/2+3/2
=（根号2+3）/2