cos（a+π/4)=-4/5=√2/2(cosa-sina), 4√2/5+cosa=sina , 32/25+8√2/5cosa+cos^2a=1-cos^2a2cos^2a+8√2/5cosa+7/25=0(√2cosa+1/25)(√2cosa+7/25)=0, cosa=-√2/50或-7√2/50sina=√62/25或√38/25tana=sina/cosa
是不是两个答案啊！负7或负7分之1
