a+tb=(4+2t,-3+t)
(a+tb)*b=2(4+2t)+(-3+t)=5+5t
又有(a+tb)*b=|a+tb||b|cos45
5+5t=根號(hào)[(4+2t)^2+(-3+t)^2]*根號(hào)(4+1)*根號(hào)2/2
25+50t+25t^2=(16+16t+4t^2+9-6t+t^2)*5/2
10+20t+10t^2=5t^2+10t+25
5t^2+10t-15=0
t^2+2t-3=0
(t+3)(t-1)=0
t=-3或t=1