题目内容：

已知sin(A+π/4)=7根号2/10,A属于(π/4,π/2)求cosA的值

优质解答

sin(A+π/4)=7√2/10,A属于(π/4,π/2)
cos(A+π/4)=-√2/10
cosA=cos[(A+π/4)-π/4]
=cos(A+π/4)cosπ/4-sin(A+π/4)sinπ/4
=(-√2/10)*(√2/2)-(7√2/10)*(√2/2)
=-4/5

追问：

你的答案肯定不对....

追答：

sin(A+π/4)=7√2/10,A属于(π/4,π/2) cos(A+π/4)=-√2/10 cosA=cos[(A+π/4)-π/4] =cos(A+π/4)cosπ/4+sin(A+π/4)sinπ/4 =(-√2/10)*(√2/2)+(7√2/10)*(√2/2) =3/5