∵a属于(π/4,3π/4),∴（π/4-a）属于(-π/2,0)又∵tan(π/4-a)= -5/12 ,∴sin(π/4-a)= -5/13,cos(π/4-a)=12/13 ⑴∵ cos(3π/2+2b)=7/25,而cos(3π/2+2b)=cos[2π-(3π/2+2b)]=cos(π/2-2b)∴cos(π/2-2b)=7/25 ...