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Tutor profile: Arturo R.

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Arturo R.
Ph.D. student in Particle Physics
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Questions

Subject: Geometry

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Question:

A certain square is to be drawn on a coordinate plane. One of the vertices must be on the origin, and the square is to have an area of 100. If all coordinates of the vertices must be integers, how many different ways can this square be drawn? (A) 4 (B) 6 (C) 8 (D) 10 (E) 12

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Arturo R.
Answer:

First, let's calculate the length of the side of the square. $$A_{square}=a^2$$, where $$a$$ is the length of the side. Now, let's try to build the square. First we need to find a point which distance from (0, 0) is 10. For this, we can use the distance formula in the plane: $$d = \sqrt{(x_2-x_1)^2 + (y_2-y_1)^2}$$ which for $$x_1=0$$ and $$y_1 = 0$$ transforms as $$d=\sqrt{(x_2)^2 + (y_2)^2}$$. The first point we are looking for is connected to the origin and therefore, its components will form a right triangle in which, the Pythagoras theorem holds, see figure 1. Then, $$x_2$$, $$y_2$$ and 10 are a Pythagorean triple. From this, $$x_2= 6$$ or $$x_2=8$$ while $$y_2= 6$$ or $$y_2=8$$. This leads us with the set of coordinates: $$(\pm 6, \pm 8)$$ and $$(\pm 8, \pm 6)$$. (A) The next step is to find the coordinates of points that lie on lines which are perpendicular to the lines that joins the origin of the coordinate system with the set of points given in (A): Let's do this for the point (6, 8). The equation of the line that join the point (6, 8) with the origin (0, 0) has the equation $$y = mx +n$$, however, we only need to find its slope in order to find a perpendicular line to it. Thus, $$m = \frac{y_2-y_1}{x_2-x_1} \\m = \frac{8-0}{6-0} \\m = 8/6$$ Then, a perpendicular line has an slope $$m_{\bot} = -\frac{1}{m} = -\frac{6}{8}$$ (perpendicularity condition of two lines). With the equation of the slope of the perpendicular line and the given point (6, 8), together with the equation of the distance we can form a system of equations to find the coordinates of two points that lie on this perpendicular line. $$m_{\bot}=\frac{6}{8} = \frac{8-y}{6-x}\\ 6(6-x)+8(8-y)=0$$ (1) $$d^2 = \sqrt{(y_o-y)^2+(x_o-x)^2} \\(10)^2=\sqrt{(8-y)^2+(6-x)^2}\\100 = \sqrt{(8-y)^2+(6-x)^2}$$ (2) This system has solutions in the coordinates (-2, 14) and (14, 2). Until here, we have three vertices of the square. Let's now find the fourth one in the same way we found the third one using the point (14,2). A line perpendicular to the line that joins the point (6, 8) and (14, 2) has an slope $$m = 8/6$$ based on the perpendicularity condition. Thus, we can form the system: $$\frac{8}{6} =\frac{2-y}{14-x} \\8(14-x) - 6(2-y) = 0$$ (1) $$100 = \sqrt{(14-x)^2+(2-y)^2}$$ (2) with solution the coordinates (8, -6) and (20, 10). If you draw a line joining the coordinates (0, 0), (6, 8), (14, 2) and (8, -6) you will get one of the squares that fulfill the conditions of the problem. By repeating this process with the coordinates in (A), the following squares are found: (0, 0), (6, 8), (14, 2), (8, -6) (0, 0), (8, 6), (14, -2), (6, -8) (0, 0), (-6, 8), (-14, 2), (-8, -6) (0, 0), (-8, 6), (-14, -2), (-6, -8) Now, notice that the equation of distance between the two points separated a distance of 10 has the trivial solution $$(\pm10, 0)$$ and $$(0, \pm10)$$. By combining this points we get the following squares: (0, 0), (10, 0), (10, 10), (0, 10) (0, 0), (0, 10), (-10, 10), (-10, 0) (0, 0), (-10, 0), (-10, -10), (0, -10) (0, 0), (0, -10), (-10, -10), (10, 0) See the attached figure. Therefore, 8 squares can be drawn

Subject: Calculus

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Question:

Consider the following function $$f(x) = 2x^3 + 9x^2 − 24x$$ (a) Find the critical numbers of $$f$$. (b) Find the open intervals on which the function is increasing or decreasing. (c) Apply the First Derivative Test to identify the relative extremum.

Inactive
Arturo R.
Answer:

(a) The critical numbers of a function are given by finding the roots of the first derivative of the function or the values where the first derivative does not exist. Since the function is a polynomial, its domain and the domain of its derivatives is $$(-\infty, \infty)$$. Thus: $$\frac{df(x)}{dx} = \frac{d(2x^3+9x^2-24x)}{dx} =6 x^2+18x -24\\6 x^2+18x -24=0\\\boxed{x=-4, x=1}$$ (b) A function $$f(x)$$ defined on an interval is monotone increasing on $$(a, b)$$ if for every $$x_1, x_2 \in (a, b): x_1<x_2$$ implies $$f(x_1)<f(x_2)$$ A function $$f(x)$$ defined on an interval is monotone decreasing on $$(a, b)$$ if for every $$x_1, x_2 \in (a, b): x_1<x_2$$ implies $$f(x_1)>f(x_2)$$ Combining the domain $$(-\infty, \infty)$$ with the critical numbers we have the intervals $$(-\infty, -4)$$, $$(-4, 1)$$ and $$(1, \infty)$$. Note that any of the points are included, in the case of the infinity it is by definition and the critical number are never included because the function monotony is not defined in the critical points, i.e. it is not monotone increasing or decreasing. Now, let's check for the monotony in each interval, for this, we check for the sign of the first derivative in each interval. Evaluating in each interval the first derivative (one point is enough), we obtain the monotony of the function to be: Increasing for $$(-\infty, -4)$$ Decreasing for $$(-4, 1)$$ Increasing for $$(1, \infty)$$ (c) From the values obtained in (a) so the relative extremum are the points $$(-4, 112)$$ and $$(1, -13)$$. The $$y$$-values are found by evaluating the critical numbers in the original function. Since the first derivative decreases after passing through $$x=-4$$ and increases after passing through the point $$x=1$$ we have: relative maximum $$(-4, 112)$$ relative minimum $$(1, -13)$$

Subject: Algebra

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Question:

Evaluate: $$-18 + 4(6 \div 2)^2$$

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Arturo R.
Answer:

To evaluate the expression, we need to follow the order of math operations: The precedence is as follows: -Parentheses (simplify inside them) -Exponents -Multiplication and Division (from left to right) -Addition and Subtraction (from left to right) In our problem: 1- $$-18+4(3)^2$$ 2-$$-18+4\cdot9$$ 3- $$-18+36$$ 4- $$18$$ Therefore, the evaluation of $$-18 + 4(6 \div 2)^2$$ gives $$18$$

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