Question #217677

(3.1) Find an expression for 12 || ~ u + ~ v || 2 + 12 || ~ u − ~ v || 2 in terms of || ~ u || 2 + || ~ v || 2

(3.2) Find an expression for || ~ u + ~ v || 2 − || ~ u − ~ v || 2 in terms of ~ u · ~ v

3.3) Use the result of (3.2) to deduce an expression for || ~ u + ~ v || 2 whenever ~ u and ~ v are orthogonal

to each other.


1
Expert's answer
2021-07-18T17:05:44-0400

(3.1). Given arbitrary vectors uu and v.v. We get


u+v2=(u+v)(u+v)||u+v||^2=(u+v)(u+v)=(u,u)+(u,v)+(v,u)+(v,v)=(u,u)+(u,v)+(v,u)+(v,v)=u2+2(u,v)+v2=||u||^2+2(u,v)+||v||^2




uv2=(uv)(uv)||u-v||^2=(u-v)(u-v)=(u,u)(u,v)(v,u)+(v,v)=(u,u)-(u,v)-(v,u)+(v,v)=u22(u,v)+v2=||u||^2-2(u,v)+||v||^2

Then



u+v2+uv2||u+v||^2+||u-v||^2=u2+2(u,v)+v2+u22(u,v)+v2=||u||^2+2(u,v)+||v||^2+||u||^2-2(u,v)+||v||^2=2u2+2v2=2||u||^2+2||v||^2

Therefore



12u+v2+12uv2=u2+v2\dfrac{1}{2}||u+v||^2+\dfrac{1}{2}||u-v||^2=||u||^2+||v||^2

(3.2).



u+v2uv2||u+v||^2-||u-v||^2=u2+2(u,v)+v2u2+2(u,v)v2=||u||^2+2(u,v)+||v||^2-||u||^2+2(u,v)-||v||^2=4(u,v)=4(u,v)

Therefore



u+v2uv2=4uv||u+v||^2-||u-v||^2=4u\cdot v



(3.3). If u and v are orthogonal to each other, then uv=0.u\cdot v=0.

Hence


u+v2uv2=0||u+v||^2-||u-v||^2=0=>u+v2=uv2=>||u+v||^2=|u-v||^2




12u+v2+12uv2\dfrac{1}{2}||u+v||^2+\dfrac{1}{2}||u-v||^2=12u+v2+12u+v2=\dfrac{1}{2}||u+v||^2+\dfrac{1}{2}||u+v||^2=u+v2=||u+v||^2=12uv2+12uv2=\dfrac{1}{2}||u-v||^2+\dfrac{1}{2}||u-v||^2=uv2=||u-v||^2=u2+v2=||u||^2+||v||^2uv2=u+v2=u2+v2||u-v||^2=||u+v||^2=||u||^2+||v||^2




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