Does A Protractor Help?

Calculus Level 2

What can we say about these two curves: y 1 = e x and y 2 = e x ? \large y_1=e^{x} \qquad \text{ and } \qquad y_2=e^{-x} \; ?

They intersect at ( 0 , 1 ) (0,1) and have an acute angle at the point of intersection They intersect at ( 0 , 1 ) (0,1) and create a right angle at the point of intersection They intersect at ( 0 , 1 ) (0,1) and have an obtuse angle at the point of intersection They do not intersect

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1 solution

Chew-Seong Cheong
May 23, 2016

If y 1 = e x y_1 = e^x and y 2 = e x y_2 = e^{-x} intersect then y 1 = y 2 y_1 = y_2 or

e x = e x e 2 x = 1 2 x = ln 1 = 0 x = 0 y 1 = y 2 = e 0 = 1 \begin{aligned} e^x & = e^{-x} \\ e^{2x} & = 1 \\ 2x & = \ln 1 = 0 \\ \implies x & = 0 \\ \implies y_1 & = y_2 = e^0 = 1 \end{aligned} .

The two curves intersect at ( 0 , 1 ) (0,1) .

The gradients of the two curves are:

{ d y 1 d x 0 = e x 0 = 1 d y 2 d x 0 = e x 0 = 1 \begin{cases} \dfrac{dy_1}{dx} \big|_0 & = e^x \big|_0 & = 1 \\ \dfrac{dy_2}{dx} \big|_0 & = -e^{-x} \big|_0 & = -1 \end{cases}

therefore, the two curves are perpendicular at the intersection.

That is:

They intersect at (0,1) and created a right angle at the point of intersection. \boxed{\text{They intersect at (0,1) and created a right angle at the point of intersection.}}

Wow, wow and wow. Very nice work :)

Hana Wehbi - 5 years ago

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