|a+b|=|a-b| |a+b|^2=|a-b|^2 a*b=0∴ a垂直b
|a-2b|^2=a^2-4ab+4b^2=4-4*0+4=8
｜a-2b｜=√8
cosa=(a-2b)*b/|b||a-2b|=-√2/2
a=135cosa=(a-2b)*b/|b||a-2b|=-√2/2能把(a-2b)*b/|b||a-2b|用数字代进去算给我看，可以吗(ab-2b^2)/(√8*1)=(0-2)/(2√2)=-√2/2