MHB Brandon's questions at Yahoo Answers regarding tangent lines

  • Thread starter Thread starter MarkFL
  • Start date Start date
  • Tags Tags
    Lines Tangent
MarkFL
Gold Member
MHB
Messages
13,284
Reaction score
12
Here are the questions:

Tangents, Normal. Need some help.?


I just recently took a test, and i am unsure of the following questions. It would be great if you provide me some explanations and workings.

1) A curve has the equation y = 2x^(2) - kx + 3, where k is a constant. The tangent at the point A passes through the point B(5,1). Find the value of k.

2) A curve has the equation y = x + x^(2). Find

i) The equation of the tangent to the curve at the point where x = a.

ii) The value(s) of a for which this tangent passes through the point P(-2,3)

iii) Hence find the possible equations of the tangent from P to the curve.

Thank you so much guys! Have a great day

I have posted a link there to this thread so the OP can view my work.
 
Mathematics news on Phys.org
Hello Brandon,

1.) We are given the curve:

$$y=2x^2-kx+3$$ where $k$ is a constant.

Let point $A$ be given by:

$$\left(x_A,y_A \right)=\left(x_A,2x_A^2-kx_A+3 \right)$$

The slope of the line passing through the points $A$ and $B$ is:

$$m=\frac{\left(2x_A^2-kx_A+3 \right)-(1)}{\left(x_A \right)-(5)}=\frac{2x_A^2-kx_A+2}{x_A-5}$$

Also, given that:

$$m=\left.\frac{dy}{dx} \right|_{x=x_A}=4x_A-k$$

We may then state:

$$\frac{2x_A^2-kx_A+2}{x_A-5}=4x_A-k$$

Multiplying through by $$x_A-5$$, we obtain:

$$2x_A^2-kx_A+2=4x_A^2-(20+k)x_A+5k$$

Combining like terms, we may arrange this as:

$$5k=2+20x_A-2x_A^2$$

Hence:

$$k=\frac{2}{5}\left(1+10x_A-x_A^2 \right)$$

2.) We are given the curve:

$$y=x+x^2$$

i) First we find the slope of the tangent line at $x=a$ is:

$$m=\left.\frac{dy}{dx} \right|_{x=a}=2a+1$$

And this tangent line must pass through the point:

$$\left(a,a+a^2 \right)$$

Thus, using the point-slope formula, we find the equation of the tangent line is given by:

$$y-\left(a+a^2 \right)=(2a+1)(x-a)$$

Distributing on the right side, we get:

$$y-\left(a+a^2 \right)=(2a+1)x-2a^2-a$$

Adding $a+a^2$ to both sides, we get the tangent line in slope-intercept form:

$$y=(2a+1)x-a^2$$

ii) If this tangent line passes through the point $(-2,3)$, then we must have:

$$3=(2a+1)(-2)-a^2$$

Now we may solve for $a$. Arranging the quadratic in $a$ in standard form, we obtain:

$$a^2+4a+5=0$$

We see the discriminant is negative, and so we conclude there are no real values of $a$ for which a tangent line to the given quadratic curve will pass through point $B$.

iii) There are no such possible tangent lines as we found in part ii).

I suspect that point $P$ was incorrectly given. I will consider the two following cases:

a) Point $P$ is supposed to be $(-2,-3)$. Then part ii) becomes:

$$-3=(2a+1)(-2)-a^2$$

Now we may solve for $a$. Arranging the quadratic in $a$ in standard form, we obtain:

$$a^2+4a-1=0$$

Application of the quadratic formula yields:

$$a=-2\pm\sqrt{5}$$

And so the two tangent lines would be given by:

$$y=\left(2\left(-2\pm\sqrt{5} \right)+1 \right)x-\left(-2\pm\sqrt{5} \right)^2=\left(-3\pm2\sqrt{5} \right)x-\left(9\pm4\sqrt{5} \right)$$

b) Point $P$ is supposed to be $(2,3)$. Then part ii) becomes:

$$3=(2a+1)(2)-a^2$$

Now we may solve for $a$. Arranging the quadratic in $a$ in standard form, we obtain:

$$a^2-4a+1=0$$

Application of the quadratic formula yields:

$$a=2\pm\sqrt{3}$$

And so the two tangent lines would be given by:

$$y=\left(2\left(2\pm\sqrt{3} \right)+1 \right)x-\left(2\pm\sqrt{3} \right)^2=\left(5\pm2\sqrt{3} \right)x-\left(7\pm4\sqrt{3} \right)$$
 
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...
Fermat's Last Theorem has long been one of the most famous mathematical problems, and is now one of the most famous theorems. It simply states that the equation $$ a^n+b^n=c^n $$ has no solutions with positive integers if ##n>2.## It was named after Pierre de Fermat (1607-1665). The problem itself stems from the book Arithmetica by Diophantus of Alexandria. It gained popularity because Fermat noted in his copy "Cubum autem in duos cubos, aut quadratoquadratum in duos quadratoquadratos, et...
I'm interested to know whether the equation $$1 = 2 - \frac{1}{2 - \frac{1}{2 - \cdots}}$$ is true or not. It can be shown easily that if the continued fraction converges, it cannot converge to anything else than 1. It seems that if the continued fraction converges, the convergence is very slow. The apparent slowness of the convergence makes it difficult to estimate the presence of true convergence numerically. At the moment I don't know whether this converges or not.
Back
Top