Question

证明:sin(-α)sin(丌-α)-tan(-α)cot(α-丌)-2cos^2(-α)+1=sin^2α

Answer 1

sin(-α)sin(π-α)-tan(-α)cot(α-π)-2cos^2(-α)+1

＝-sinαsinα+tanαcotα-2cos^2(α)+1------------说明：-sinαsinα-cos^2(α)=-1,tanαcotα=1

=1-cos^2(α)

=sin^2α