Question #213875

Suppose u; v is the element of V . Prove that || au + bv || = || bu + av || for all a, b is the element of R if and only if || u || = || v || .


1
Expert's answer
2021-07-08T05:01:57-0400

Suppose that V=RnV={\mathbb{R}}^n. Then, we have: u=(u1,...un)u=(u_1,...u_n) and v=(v1,...,vn)v=(v_1,...,v_n). We have: (au+bv)=(au1+bv1,...,aun+bvn)(au+bv)=(au_1+bv_1,...,au_n+bv_n) and (bu+av)=(bu1+av1,...,bun+avn)(bu+av)=(bu_1+av_1,...,bu_n+av_n). We must obtain the equality: (au1+bv1)2+...+(aun+bvn)2=(bu1+av1)2+...+(bun+avn)2\sqrt{(au_1+bv_1)^2+...+(au_n+bv_n)^2}=\sqrt{(bu_1+av_1)^2+...+(bu_n+av_n)^2}. Simplify the left side: (a2u12+2abu1v1+b2v12+...+a2un2+2abunvn+b2vn2.\sqrt{(a^2u^2_1+2abu_1v_1+b^2v^2_1+...+a^2u^2_n+2abu_nv_n+b^2v^2_n}. Right side has the form: (bu1+av1)2+...+(bun+avn)2=b2u12+2abu1v1+a2v12+...+b2un2+2abunvn+a2vn2\sqrt{(bu_1+av_1)^2+...+(bu_n+av_n)^2}=\sqrt{b^2u^2_1+2abu_1v_1+a^2v_1^2+...+b^2u^2_n+2abu_nv_n+a^2v_n^2}

Since by assumption we have: u12+...+un2=v12+...+vn2\sqrt{u_1^2+...+u_n^2}=\sqrt{v_1^2+...+v_n^2} . Using the latter we observe that left and right side are equal. Thus, the equality au+bv=bu+av||au+bv||=||bu+av|| holds. We showed that if u=v||u||=||v||, then the equality au+bv=bu+av||au+bv||=||bu+av|| holds. Otherwise, we can set a=1,b=0a=1,b=0 and obtain u=v.||u||=||v||.


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