### Video Transcript

If the vector π is equal to three
π’ minus five π£, π is equal to ππ’ plus five π£, and the cross product of π and
π is equal to 50π€, find the value of π.

To answer this question, weβre
going to need to recall what we mean by the cross product of two vectors. Essentially, itβs a way of
multiplying two vectors. Unlike the dot product, where we
get a scalar quantity, when we find the cross product of two vectors, we get a
vector. And we say that if π is the
three-dimensional vector given by π one, π two, π three and π is the vector π
one, π two, π three. Then the cross product of π and π
is equal to the vector π two π three minus π three π two, π three π one minus
π one π three, and π one π two minus π two π one.

Now, this isnβt a particularly easy
formula to remember. And if youβre struggling, you might
want to recall that itβs really just the determinant of a three-by-three matrix. So now, we have a formula. Letβs define π one, π two, π
three and π one, π two, π three. π one is the horizontal component
for π. Itβs three. π two is the vertical
component. Thatβs negative five. And thereβs no π€-component. So π three is equal to zero. π one is equal to π. π two is equal to five. And once again, π three is equal
to zero.

The first element of the cross
product of these two vectors then is π two π three. So thatβs negative five times zero
minus π three π two, which is zero multiplied by five. The second element is π three π
one, which is zero times π, minus π one π three, which is three times zero. And the third element is π one π
two, thatβs three times five, minus π two π one, thatβs negative five multiplied
by π. The first and second elements
simplify to zero. And the third element of the cross
product of our two vectors becomes 15 plus five π. And thatβs because we have minus
negative five. So we get plus five.

Now, if we go back to the question,
we see that the cross product of π and π is given to us. Weβre told itβs 50π€. Well, another way to represent that
is using these sort of triangular brackets. The vector π crossed with π is
zero, zero, 50. And we see then that, for the
vectors to be equal, 50 must be equal to 15 plus five π. Weβll solve by subtracting 15 from
both sides. And that gives us 35 equals five
π. We then divide both sides of this
equation by five. And we get seven equals π.

And so if π is equal to three π’
minus five π£, π is equal to ππ’ plus five π£, and the cross product of π and π
is 50π€, π must be equal to seven.