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Homework Statement
Which of the following functions is not an inner product in [tex]\mathbb{R}^2[/tex], where u=(a, b), v=(c,d)?
a. <u,v> = ac+bd
b. <u,v> = 4ad+7bc
c. <u,v> = ac/2+bd/3
d. <u,v> = 2ac+9bd
Homework Equations
None
The Attempt at a Solution
u x v = det(u v) = ad-bc
I don't see how ANY of these answers could be right. Any ideas?