Video Transcript
Find an equation of the tangent to
the curve 𝑥 is equal to sin of 𝜋 times 𝑡 and 𝑦 is equal to 𝑡 squared plus 𝑡 at
the point zero, two.
The question gives us a pair of
parametric equations. And it wants us to find an equation
of the tangent to the curve represented by this pair of parametric equations at the
point zero, two. To find an equation of a line, we
need to find the slope of that line. Since we want our line to be a
tangent to the curve, we can find the slope by using d𝑦 by d𝑥. However, we’re given 𝑥 and 𝑦 as a
pair of parametric equations.
We recall if 𝑥 is a function of 𝑡
and 𝑦 is a function of 𝑡, then we can find d𝑦 by d𝑥 by dividing the derivative
of 𝑦 with respect to 𝑡 by the derivative of 𝑥 with respect to 𝑡. So we set 𝑓 of 𝑡 to be sin of
𝜋𝑡 and 𝑔 of 𝑡 to be 𝑡 squared plus 𝑡. Then by differentiating these with
respect to 𝑡, we can find an expression for d𝑦 by d𝑥 . We start by calculating
d𝑦 by d𝑡. That’s the derivative of 𝑡 squared
plus 𝑡 with respect to 𝑡.
We can differentiate this using the
power rule for differentiation. We multiply by the exponent and
then reduce the exponent by one. This gives us two 𝑡 plus one. We can do the same to find d𝑥 by
d𝑡. It’s the derivative of sin of 𝜋𝑡
with respect to 𝑡. To differentiate this, we recall
that, for any constant 𝑎, the derivative of the sin of 𝑎𝑡 with respect to 𝑡 is
equal to 𝑎 times the cos of 𝑎𝑡. Therefore, since 𝜋 is just a
constant, the derivative of the sin of 𝜋𝑡 with respect to 𝑡 is just equal to 𝜋
times the cos of 𝜋𝑡.
Now that we found d𝑦 by d𝑡 and
d𝑥 by d𝑡, we can find an expression for d𝑦 by d𝑥. It’s equal to two 𝑡 plus one
divided by 𝜋 times the cos of 𝜋𝑡. We see that our expression for d𝑦
by d𝑥 is in terms of 𝑡. This means it tells us the slope of
the tangent to the curve at the point 𝑡. However, we want to find the slope
of the tangent at the point zero, two. So we need to find the value of 𝑡
which gives us the point zero, two. The curve being at the point zero,
two is the same as saying its 𝑥-coordinate is equal to zero and its 𝑦-coordinate
is equal to two.
To find the values of 𝑡 where our
curve is at the point zero, two, we’ll set our 𝑦-value equal to two, giving us the
equation two is equal to 𝑡 squared plus 𝑡. We then subtract two from both
sides of this equation, giving us that zero is equal to 𝑡 squared plus 𝑡 minus
two. And we see in this case we can
fully factor the quadratic. It’s equal to 𝑡 plus two
multiplied by 𝑡 minus one.
Since we want this to be equal to
zero, one of the factors must be equal to zero. Solving these gives us 𝑡 is equal
to negative two or 𝑡 is equal to one. We need to check that these values
of 𝑡 give us an 𝑥-coordinate of zero. We see when 𝑡 is equal to one, 𝑥
is equal to sin of 𝜋 times one, which is just equal to zero. And we also see when 𝑡 is equal to
negative two, 𝑥 is equal to the sin of negative two 𝜋, which is also equal to
zero. So we have two different values of
𝑡 at which our parametric curve is at the point zero, two.
At first, this might be
confusing. We have several different values of
𝑡. We don’t know which one we should
pick. Sometimes our values of 𝑡 are
restricted. However, that’s not given in this
case. In this case, we’re asked to find
an equation of the tangent. So it doesn’t actually matter which
value of 𝑡 we pick. We will always get an equation for
the tangent. However, it’s worth noting at this
point that the value of 𝑡 that you choose could possibly give completely different
equations for the tangent.
For example, consider a parametric
curve which gives a figure-eight pattern as shown in the sketch. Then, if we were asked to find a
tangent at the origin to this curve, we can see that there are two
possibilities. Both lines share a slope with
different parts of the curve at the origin. In fact, both of these are tangent
lines to the curve at the origin. And which tangent line you get
would depend on which value of the parameterization variable you choose.
In our case, since we’re just asked
to find an equation of the tangent, it doesn’t matter which value of 𝑡 we
choose. Let’s start by checking 𝑡 is equal
to one. We have the slope of our curve when
𝑡 is equal to one is equal to two times one plus one divided by 𝜋 times the cos of
𝜋 times one, which we can calculate to give us negative three divided by 𝜋.
To be thorough, let’s check what
the value of our slope would’ve been if we had chosen 𝑡 is equal to negative two
instead. We would’ve gotten a slope of two
times negative two plus one all divided by 𝜋 times the cos of 𝜋 times negative
two. And we would’ve gotten, again, the
slope to be equal to negative three divided by 𝜋. So in this case, we can see the
slope is the same regardless of which value of 𝑡 we would’ve gotten. This means our equation of the
tangent will be the same in both cases.
To help us find the equation of our
tangent line to the curve at the point zero, two, we recall the equation of a line
which passes through the point 𝑥 one, 𝑦 one with a slope of 𝑚 is given by 𝑦
minus 𝑦 one is equal to 𝑚 times 𝑥 minus 𝑥 one.
We already found slope of our
tangent line. It’s equal to negative three
divided by 𝜋. And we already know our tangent
line passes through the point zero, two. So we’ll set 𝑥 one equal to zero
and 𝑦 one equal to two. This gives us 𝑦 minus two is equal
to negative three over 𝜋 times 𝑥 minus zero. Simplifying this and rearranging to
make 𝑦 the subject gives us 𝑦 is equal to negative three over 𝜋 times 𝑥 plus
two.
Therefore, we’ve shown that an
equation of the tangent to the curve 𝑥 is equal to the sin of 𝜋𝑡 and 𝑦 is equal
to 𝑡 squared plus two at the point zero, two is 𝑦 is equal to negative three over
𝜋 𝑥 plus two.
