a⊥b→ab=0
|a|=3,
|c|=√(4a+3b)^2=√16a^2+9b^2+24ab=√16*9+9*16=12√2
ac=a(4a+3b)=4a^2+3ab=4*9=36
cos=ac/(|a|*|c|)=36/(3*12√2)= √2/2
故=45度
即ac夹角为45度.