# Adding three vectors, why is this not right?

http://postimage.org/image/gcb728073/ [Broken]

A=47.2 at 54.3deg above the x axis, B= 23.5 at 34.4 above the -x axis, C=30.5 along the -y axis.

A+B+C=(Ax+Bx+Cx)+(Ay+By+Cy) = R

R=√(Rx2+Ry2) = 22.63

θ=tan-1Ry/Rx = 68.9deg

I just thought of the possibility that c may be at 90 deg instead of 270. As in pointing upwards along the neg y axis. Is there something i missed?

## The Attempt at a Solution

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## Answers and Replies

Homework Helper
Gold Member
What values and signs are you getting for the x and y components of the three vectors? C points down with a magnitude of 30.5.

Ax=Acosθ=47.2cos(54.3) = 27.5

Ay=Asinθ=47.2sin(54.3) = 38.3

Bx=Bcosθ=23.5cos(180-34.4) = -19.4

By=Bsinθ=23.5sin(180-34.4)= 13.3

Cx=Ccosθ=30.5cos(270)=0

Cy=Csinθ=30.5sin(270)=-30.5

Rx= 27.5-19.4+0=8.1
Ry=38.3+13.3-30.5=21.1

R=√(8.12)+(21.12)=22.6

θ=tan-1(21.1/8.1)=68.9deg

Ive tried rounding to different numbers of decimal places and not rounding till the end but I still had no success and I dont know why.

Homework Helper
Ax=Acosθ=47.2cos(54.3) = 27.5

Ay=Asinθ=47.2sin(54.3) = 38.3

Bx=Bcosθ=23.5cos(180-34.4) = -19.4

By=Bsinθ=23.5sin(180-34.4)= 13.3

Cx=Ccosθ=30.5cos(270)=0

Cy=Csinθ=30.5sin(270)=-30.5

Rx= 27.5-19.4+0=8.1
Ry=38.3+13.3-30.5=21.1

R=√(8.12)+(21.12)=22.6

θ=tan-1(21.1/8.1)=68.9deg

Ive tried rounding to different numbers of decimal places and not rounding till the end but I still had no success and I dont know why.

The problem doesn't say to find A+B+C. It says to find A-B+C.