Use Exercise 91 to show that v and w are orthogonal if and only if v w = v + w. Data From Exercise

Use Exercise 91 to show that v and w are orthogonal if and only if ΙΙv − wΙΙ = ΙΙv + wΙΙ.

Data From Exercise 91

Prove that ΙΙv + wΙΙ² − ΙΙv − wΙΙ² = 4v · w.