①已知cos(π/4-a)=3/5,sin(3π/4+b)=5/13,其中π/4
问题描述:
①已知cos(π/4-a)=3/5,sin(3π/4+b)=5/13,其中π/4
答
1.因为(3π/4+b)-(π/4-a)=b+a+π/2所以cos[(a+b)+π/2]=cos(a+b)*cosπ/2-sin(a+b)*sinπ/2=-sin(a+b)所以sin(a+b)=-cos[(a+b)+π/2]=-cos[(3π/4+b)-(π/4-a)] =-[cos(3π/4+b)*cos(π/4-a)+sin(3π/4+b...