Question

Which of the following points below represents (- 2, - (5pi)/4) the polar grid?

Answer 1

Answer:

Well, when we have a point in rectangular coordinates (x, y) and we want to write it in polar coordinates (r, θ) the rule we use is:

r = √(x^2 + y^2)

θ = Atg(Iy/xI) + 90°*(n - 1)

where n is number of the quadrant where our point is.

For example, if we are on the second quadrant, we use n = 2.

Now, if instead, we have the point (r, θ) and we want to rewrite it in rectangular coordinates, then the transformation is:

x = r*cos(θ)

y = r*sin(θ)

Now we have the point (- 2, -(5pi)/4) because the first part is negative, this number can not be in polar coordinates (r can not be negative) then we have:

x = -2

y = -(5*pi)/4

Using the first relation, we can find that:

r = √( (-2)^2 + (-5*3.14/4)^2) = 4.4

(rememer that the point (- 2, -(5pi)/4) is on the third quadrant, then we will use n = 3.)

θ = Atg(-(5*pi)/4/-2) + 90°*(3 - 1)= Atg((5*pi)/2) + 180° = 82.7° + 180° = 262.7°

Then the point that represents  (- 2, - (5pi)/4) in polar coordinates is the point:

(4.4, 262.7°)

If instead of degrees, the angle part is written in radians, we have:

180° = 3.14 radians.

262.7° = x radians.

then:

x/3.14 = (262.7)/180

x =  3.14*(262.7)/180 = 4.58 radians.

then the point will be:

(4.4, 4.58 radians)