1. Positive Definite Property:
⟨x,x⟩=2x1x1−x1x2−x2x1+2x2x2
=(x12+x22)+(x1−x2)2≥0 The value equals zero if and only if both summands are zero, i.e., when x1=x2=0
For any x∈V,⟨x,x⟩≥0; and ⟨x,x⟩=0 if and only if x=0.
2. Symmetric Property
⟨x,y⟩=2x1y1−x1y2−x2y1+2x2y2
=2y1x1−y1x2−y2x1+2y2x2=⟨y,x⟩ 3. Linearity
⟨ax+by,z⟩
=2(ax1+by1)z1−(ax1+by1)z2−(ax2+by2)z1
+2(ax2+by2)z2
=a(2x1z1−x1z2−x2z1+x2z2)
+b(2y1z1−y1z2−y2z1+y2z2)
=a⟨x,z⟩+b⟨y,z⟩ Then ⟨x,y⟩ is an inner product on V.
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