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

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

人气：421 ℃　时间：2020-01-25 04:14:03

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你的答案肯定不对....sin(A+π/4)=7√2/10,A属于(π/4,π/2)cos(A+π/4)=-√2/10cosA=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