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 ||.
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Expert's answer
2021-07-08T05:01:57-0400
Suppose that V=Rn. Then, we have: u=(u1,...un) and v=(v1,...,vn). We have: (au+bv)=(au1+bv1,...,aun+bvn) and (bu+av)=(bu1+av1,...,bun+avn). We must obtain the equality: (au1+bv1)2+...+(aun+bvn)2=(bu1+av1)2+...+(bun+avn)2. Simplify the left side: (a2u12+2abu1v1+b2v12+...+a2un2+2abunvn+b2vn2. Right side has the form: (bu1+av1)2+...+(bun+avn)2=b2u12+2abu1v1+a2v12+...+b2un2+2abunvn+a2vn2
Since by assumption we have: u12+...+un2=v12+...+vn2 . Using the latter we observe that left and right side are equal. Thus, the equality ∣∣au+bv∣∣=∣∣bu+av∣∣ holds. We showed that if ∣∣u∣∣=∣∣v∣∣, then the equality ∣∣au+bv∣∣=∣∣bu+av∣∣ holds. Otherwise, we can set a=1,b=0 and obtain ∣∣u∣∣=∣∣v∣∣.
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