what is this thing

[red and white kop!]

i am confused by this sentence in my textbook:
when a section of the curve y=f(x) is reflected in Oy, the equation of that part of the curve becomes y=-f(x)
is this correct?
i take it that Oy is the y axis (god knows why), and in that case isn't a reflection in the y axis equal to y=f(-x)?

 

[soroban]

Hello, red and white kop!!

i am confused by this sentence in my textbook:

When a section of the curve $\displaystyle y=f(x)$ is reflected in $\displaystyle Oy$,
the equation of that part of the curve becomes $\displaystyle y=-f(x)$

is this correct?
i take it that $\displaystyle Oy$ is the y-axis (god knows why), . ... I agree!
and in that case, isn't a reflection in the y axis equal to $\displaystyle y=f(-x)$ ? . ... Yes!

It's probably a typo . . .

Or ... they may be silly enough to let $\displaystyle Oy$ represent: $\displaystyle y = 0$
. . which is a truly stupid way to say "x-axis".

 

[red and white kop!]

thank you