Question

设向量a=(x,-3),b=(-1,1),若|a-b|=5,则实数x的值为

Answer 1

a-b=(x-(-1),-3-1)=(x+1,-4)

|a-b|=√（x+1）^2+(-4)^2=√（x^2+2x+10）=5

x^2+2x+10=25

x^2+2x-15=0

(x-3)(x+5)=0

x=3 or x=-5