Question

高一数学题！！新课标滴！！急急急急急急急急急~~！！！~！

化简 （sin1970-sin1990)(sin1970+sin1990)=？？

Answer 1

（sin1970-sin1990)(sin1970+sin1990)=[sin(1980-10)-sin(1980+10)]×[sin(1980-10)+sin(1980+10)]=[sin1980cos10-sin10cos1980-sin1980cos10-sin10cos1980]×[sin1980cos10-sin10cos1980 +sin1980cos10+sin10cos1980]=[-2sin10cos1980]×[2sin1980cos10]=-[2sin10cos10]×[2sin1980cos1980]
=-sin20sin3960

Answer 2

(sin1970)^2-(sin1990)^2

Answer 3

(sin1970)²-(sin1990)²=sin²（1970)²-sin²（1990)²=sin²[（1970)²-（1990)²]=sin²[(1970-1990)(1970+1990)]=sin²(-79200)=-sin²79200

Answer 4

sin我不知道这是啥！

但我知道：(a-b)(a+b)=a方-b方