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## Homework Statement

Two charged balls are placed in point A and B and the distance between them is 9,54cm. Each of the balls are charged with 8,0 x 10^-8 C. Find the scalar value and direction of the electric field in point C placed 5 cm from A and 6 cm from B.

## Homework Equations

Cosine Rule: a^2 = b^2 + c^2 - 2ab x cosθ

E = K

_{e}x (q/r^2)

## The Attempt at a Solution

[/B]

I have calculated the electric field of A and B.

E

_{A}= 287680 N/C

E

_{B}= 199778 N/C

After that I used cosine rule to calculate E

_{C}and got E

_{C}= 255341 N/C. This is correct.

But when I try finding the direction I calculate the angle between E

_{C}and E

_{A}(β) like this:

β = cos^-1 ((E

_{A}^2 + E

_{B}^2-E

_{C}^2)/(2E

_{A}E

_{B})) = 60 degrees

The solution says that I should calculate the angle between E

_{C}and E

_{A}(β), but that the equation looks like this:

β = cos^-1 ((E

_{C}^2 + E

_{A}^2-E

_{B}^2)/(2E

_{C}E

_{A})) = 42,65 degrees

Why am I supposed to use that equation?