Sin6a+sin7a-sin8a-sin9a / cos6a+cos7a+cos8a+cos9a ПОМОГИТЕ ПОЖАЛУЙСТА ))))

[latex] \frac{sin6a+sin7a-sin8a-sin9a}{cos6a+cos7a+cos8a+cos9a} = \frac{(sin6a-sin8a)+(sin7a-sin9a)}{(cos6a+cos8a)+(cos7a+cos9a)} =\\ \\ = \frac{(2*sin \frac{6a-8a}{2} *cos \frac{6a+8a}{2} )+(2*sin \frac{7a-9a}{2} *cos \frac{7a+9a}{2} )}{(2*cos \frac{6a-8a}{2} *cos \frac{6a+8a}{2} )+(2*cos \frac{7a-9a}{2} *cos \frac{7a+9a}{2} )} = \\ \\ =\frac{(2*sin(-a)*cos7a)+(2*sin(-a)cos8a)}{(2*cos(-a)*cos7a)+(2*cos(-a)*cos8a)} = \frac{-2sinacos7a-2sinacos8a}{2cosacos7a+2cosacos8a} = [/latex] [latex] = \frac{-2sina(cos7a+cos8a)}{2cosa(cos7a+cos8a)} = - \frac{sina}{cosa} =-tga[/latex]