设向量a=(x,y),则,x^2+y^2=10^2=100.(1)又∵向量a∥向量b,∴-3x-4y=0.x=(-4/3)y.将x值代入(1)式,化简,得：25y^2=900.y^2=900/25.y=±30/5.∴∴x=(-4/3)*(±30/5).∴向量a=(-120/15,30/5)；或,向量a=(-120/15,-30/5)...