How to color only the numerator in \binom{n+1}{2} in Manim?

[mrs-isi]

Hi everyone,

I’m working on a didactic Manim scene where I’m transitioning from a specific binomial identity to a general one using symbolic expressions.

One of the expressions I use is: binom_n_2 = MathTex(r"\binom{n+1}{2}", font_size=30)

Problem:
I want to highlight only the numerator (n+1) in this binomial coefficient. For example: self.play(binom_n_2[1].animate.set_color(GOLD))
But this throws an IndexError or doesn’t isolate just the numerator — because the MathTex object only contains a single submobject for the entire \binom{n+1}{2}. When I do something like: for i, part in enumerate(binom_n_2): print(i, part) it only prints one part (index 0), so accessing binom_n_2[1] fails.

What I've already tried:

1. Splitting the LaTeX manually into pieces like: teil1 = MathTex(r"\binom{", font_size=30) teil2 = MathTex(r"n+1", font_size=30).set_color(GOLD) teil3 = MathTex(r"}{2}", font_size=30)
   but it doesn't work.
2. Using substrings_to_isolate=["n+1"] inside the MathTex call: binom_n_2 = MathTex( r"\binom{n+1}{2}", substrings_to_isolate=["n+1"], font_size=30 ) but surprisingly, this does not work inside the binomial: the n+1 part is not separately accessible, and it still appears as a single group.
3. Directly passing separate arguments to MathTex: MathTex(r"\binom{", r"n+1", r"}{2}") binom[1].set_color(GOLD) but this crashes LaTeX or fails to render correctly (Missing } inserted error).

Goal:
I want to synchronously highlight two expressions.

1. the numerator of the binomial \binom{n+1}{2}
2. the (n+1) in "(n+1)-te Zeile"
   The second one woks with this n1_zeile = MathTex(r"(", r"n+1", r")\text{-te Zeile}", font_size=32, color=WHITE) followed by this self.play( Transform(fünfzehn, binom_n_2), n1_zeile[1].animate.set_color(GOLD), # ✅ works! binom_n_2[1].animate.set_color(GOLD), # ❌ fails or ignored! run_time=1.5 )

My question:
How can I safely and correctly color and than Indicate only the numerator (n+1) in a binomial coefficient and how can I ensure that happens in sync with another highlight (as in my self.play(...))?

Any suggestions or examples would be super appreciated. Thanks!
Isa

[uwezi]

best would be if you came over to Discord and presented a complete and runable short code snippet, instead of loose bits...
https://docs.manim.community/en/stable/faq/general.html?highlight=discord#where-can-i-find-more-resources-for-learning-manim

In general you cannot run two different animations on the same object at the same time as you are trying. Secondly the most flexible way to modify MathTex() and Tex() objects is by self determining the corresponding indices of the respective parts.

Splitting a MathTex() string works only as long as all parts are correct LaTeX code by themselves, which parts of a \frac{}{} or \binom are not.

Using index_labels() you can identify the individual indices

class binomnom(Scene):
    def construct(self):
        binom_n_2 = MathTex(r"\binom{n+1}{2}", font_size=30)
        # uncomment for index labels
        #self.add(binom_n_2)
        #self.add(index_labels(binom_n_2[0]).set_color(RED))
        #return
        self.play(Write(binom_n_2))
        self.play(
            binom_n_2[0][1:4].animate.set_color(PURE_RED),
            run_time=1.5
        )
        self.wait()

binomnom.mp4