# Need Help solving for Y

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• Sep 17th 2013, 04:53 PM
oshae993
Need Help solving for Y
Question:

COSΘ= x/r COSΘ= -7/25

Its in Quadrant III so X and Y are Negative

X= -7 R= 25

r=(x^2 + y^2)^1/2

25=((-7)^2 + y^2)^1/2
_______________________________________
My Work

25=((-7)^2 + y^2)^1/2

25=(49 + y^2)^1/2

25= 7+y

18= y which is incorrect

The math lab says this is the correct answer Y=-24 but i get something different. Can anyone help me solve this or explain where i went wrong.
• Sep 17th 2013, 05:45 PM
HallsofIvy
Re: Need Help solving for Y
Quote:

Originally Posted by oshae993
Question:

COSΘ= x/r COSΘ= -7/25

Its in Quadrant III so X and Y are Negative

X= -7 R= 25

r=(x^2 + y^2)^1/2

25=((-7)^2 + y^2)^1/2
_______________________________________
My Work

25=((-7)^2 + y^2)^1/2

25=(49 + y^2)^1/2

25= 7+y

This is your error "(a^2+ b^2)^{1/2}" is NOT (a^2)^{1/2}+ (b^2)^{1/2}

Instead square both sides of 25= (49+ y^2)^{1/2} to get 625= 49+ y^2, y^2= 625- 49= 576.

Quote:

18= y which is incorrect

The math lab says this is the correct answer Y=-24 but i get something different. Can anyone help me solve this or explain where i went wrong.
• Sep 17th 2013, 06:03 PM
oshae993
Re: Need Help solving for Y
Attachment 29185

Wow how did i do that. Thank you so much for clearing that up.