What if the angle between these two unit vectors? We know that $\vec{i}$ and $\vec{j}$ are unit vectors along $x$-axis and $y$-axis respectively. So angle between them is $90°$.
But let $\vec{a}$ and $\vec{b}$ be unit vectors such that $\vec{a}-\sqrt{2}\vec{b}$ is also a unit vector? How do we determine the angle between $\vec{a}$ and $\vec{b}$ ?
 A: If $a - \sqrt{2} b$ is a unit vector, then the inner product
\begin{equation*}
\langle a - \sqrt{2}b, a - \sqrt{2}b \rangle = ||a - \sqrt{2} b||^{2} = 1.
\end{equation*}
My suggestion would be to work out the inner product, first using bilinearity, and then using the relationship between the inner product of two vectors, their magnitudes, and their angles.
A: The vector $a-\sqrt{2}b$ is unit so $|a-\sqrt{2}b|^2=1$ that is
$$1=(a-\sqrt{2}b).(a-\sqrt{2}b)=a.a-2\sqrt{2}a.b+2b.b=2-2\sqrt{2}a.b$$
this concludes that $a.b=\dfrac{\sqrt{2}}{4}$. The angle between $a$ and $b$ is
$$\cos\theta=\frac{a.b}{|a||b|}=\frac{a.b}{|a||b|}=\dfrac{\sqrt{2}}{4}$$
finally
$$\color{blue}{\theta=\arccos\dfrac{\sqrt{2}}{4}}$$
A: The vector $a-\sqrt{2}b$ is unit vector so $|a-\sqrt{2}b|^2=1$ therefore
$$1=(a-\sqrt{2}b).(a-\sqrt{2}b)=a.a-2\sqrt{2}a.b+2b.b=3-2\sqrt{2}a.b$$
Implies that $a.b=\dfrac{2}{2\sqrt{2}}$. So angle between $a$ and $b$ is
$$\cos\theta=\frac{a.b}{|a||b|}=\frac{a.b}{|a||b|}=\dfrac{1}{\sqrt{2}}$$
finally
$$\color{green}{\theta=\arccos\dfrac{1}{\sqrt{2}} = π/4}$$
