Question

化简sin(-a-5π)cos(a-π/2)-tan(a-3π/2)xtan(2π-a)

cos（a-π/2）=sin a？！

Answer 1

=-sin(a+π)cos(a-π/2)-tan(a-π/2)tan(-a)
=sinasina+tan(a-π/2)tana
=(sina)^2-cotatana
=(sina)^2-1
=-cos2^(a)

追问

cos（a-π/2）=-cos（π/2-a)能这样不？
cos(a-π/2)这我不会变成sin a啊。。。求解。