Consider the tangent line to the curve y = 2sin(x) at the point (π/6, 1). (a) Find the unit vector that lies in the first quadrant that is parallel to the tangent line. i + j (b) Find the unit vector that lies in the fourth quadrant that is perpendicular to the tangent line. i + j

Respuesta :

Answer:

u = Cos 60° i + Sin 60° j = (1/2) i + (√3/2) j

v = Cos 30° i - Sin 30° j = (√3/2) i - (1/2) j

Step-by-step explanation:

a) If y = 2sin(x)

y' = m = (2sin (x))' = 2cos(x)

Given

x = π/6

⇒ y' = m = 2cos(π/6) = √3

then we apply

∅ = tan⁻¹(√3) = 60°

If u is the unit vector that lies in the first quadrant that is parallel to the tangent line we have

u = Cos 60° i + Sin 60° j = (1/2) i + (√3/2) j

b) We apply

(1/2, √3/2).(a, -b) = 0

⇒ (a/2) - (√3/2)b = 0

⇒ a = √3b

if b = 1/2

a = √3/2

v = Cos 30° i - Sin 30° j = (√3/2) i - (1/2) j

v is the unit vector that lies in the fourth quadrant that is perpendicular to the tangent line. i + j

a) The unit vector that lies in the first quadrant that is parallel to the tangent line is [tex]\vec u = \frac{1}{4}\,\hat{i} + \frac{\sqrt{3}}{4}\,\hat{j}[/tex].

b) The unit vector that lies in the fourth quadrant that is perpendicular to the tangent line is [tex]\vec u = \frac{\sqrt{3}}{2}\,\hat{i} -\frac{1}{2}\,\hat{j}[/tex].

Determination - Unit vector associated to a line tangent to the curve

a) Unit vector in the first quadrant

By calculus we know that that the line tangent to the curve is represented by the first derivative evaluated at the point given on statement:

[tex]m = 2\cdot \cos x[/tex] (1)

[tex]m = 2\cdot \cos \frac{\pi}{6}[/tex]

[tex]m = \sqrt{3}[/tex]

By trigonometry we know that direction of the tangent line is represented by tangent function, then we have the following expression:

[tex]\vec r = \hat{i} + \sqrt{3}\,\hat{j}[/tex] (2)

The unit vector is found by dividing (2) by its magnitude:

[tex]\vec u = \frac{1}{4}\,\hat{i} + \frac{\sqrt{3}}{4}\,\hat{j}[/tex] [tex]\blacksquare[/tex]

The unit vector that lies in the first quadrant that is parallel to the tangent line is [tex]\vec u = \frac{1}{4}\,\hat{i} + \frac{\sqrt{3}}{4}\,\hat{j}[/tex]. [tex]\blacksquare[/tex]

b) Unit vector in the fourth quadrant

By analytical geometry, the relationship between the line tangent to the curve ([tex]m[/tex]) and the line perpendicular to that ([tex]m_{\perp}[/tex]) is:

[tex]m_{\perp} = -\frac{1}{m}[/tex] (3)

If we know that [tex]m = \sqrt{3}[/tex], then the slope of the line perpendicular to the line:

[tex]m_{\perp} = -\frac{\sqrt{3}}{3}[/tex]

Now we use the same approach taken for the previous point:

[tex]\vec r = \hat{i} - \frac{\sqrt{3}}{3} \,\hat{j}[/tex] (4)

[tex]\vec u = \frac{\sqrt{3}}{2}\,\hat{i} -\frac{1}{2}\,\hat{j}[/tex] [tex]\blacksquare[/tex]

The unit vector that lies in the fourth quadrant that is perpendicular to the tangent line is [tex]\vec u = \frac{\sqrt{3}}{2}\,\hat{i} -\frac{1}{2}\,\hat{j}[/tex]. [tex]\blacksquare[/tex]

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