First slide

Equation of line in Straight lines

Question

If O is the origin and the coordinates of A and B are $(x_{1}, y_{1})$ and $(x_{2}, y_{2})$ respectively then $O A \times O B \cos ∠ A O B$ is equal to

Moderate

A

$x_{1} y_{1} + x_{2} y_{2}$
B

$x_{1} x_{2} + y_{1} y_{2}$
C

$x_{1} y_{2} + x_{2} y_{1}$
D

$x_{1} x_{2} - y_{1} y_{2}$

Solution

$2 O A \times O B \cos ⌊A O B = (O A)^{2} + (O B)^{2} - (A B)^{2}$

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