已知向量a=（4,-3） b=（2,1）,若a+tb与b的夹角为45度,则实数t=_______

a+tb=(4+2t,-3+t)(a+tb)*b=2(4+2t)+(-3+t)=5+5t又有(a+tb)*b=|a+tb||b|cos455+5t=根号[(4+2t)^2+(-3+t)^2]*根号(4+1)*根号2/225+50t+25t^2=(16+16t+4t^2+9-6t+t^2)*5/210+20t+10t^2=5t^2+10t+255t^2+10t-15=0t^2+2t-...