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1. I need help with question 3iii

I know it's a parallelogram, and the co-ordinates of the four verticles are:
A(5,2) B(1,1) C(2,4) D(6,5)

AB is parallel to CD
and AC is parallel to BD

for question 3iii)

I'm not sure how to find the internal angles?

This is what I've done to find the first angle (the angle at A)

vector AB= (1,1) - (5,2) = (-4, -1)
vector AC= (2,4) - (5,2) = (-3,2)

dot product = 10

square root: (4^2+1^2) = sqrt (17)
square root: (3^2+2^2)= sqrt (13)

cos^-1 (10/ [sqrt (17)*sqrt (13)]

but it was the wrong answer

(I haven't worked out the second angle yet because I realized I'm doing it wrong)

the answers are meant to be 57.5 degrees and 122.5 degrees.

what am i doing wrong?
2. AC is a diagonal, not a side?
3. oh thanks! I'm meant to do AB and AD vectors

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Updated: April 11, 2013
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