tan((3π/2 )+a)=4/3=-cota=4/3 所以cota=-4/3 因为a∈（3π/2 ,2π）,所以sina=-4/5,cosa=3/5 所以cos(a-(3π/4))=cosacos3π/4 +sinasin3π/4 =3/5*（-√2/2）+（-4/5）√2/2 =-7√2/10