Cambria Math Font download valid for Indesign

No, you shouldn't have to purchase/download separately to use them in InDesign — as long as you own any of the Microsoft software programs listed here: https://learn.microsoft.com/en-us/typography/font-list/cambria-math

• MS Windows 10 or 11 (Windows)
• MS Office 365 (Windows)

When fonts are installed into the main font folder on Windows computers — C:/Windows/Fonts — they are available to all software programs running on the computer, including Adobe InDesign and the rest of Creative Suite.

But if you're on a Mac, you might have to download a copy of the fonts and install them on your Mac.

If Cambria doesn't end up working for you, try downloading and installing (on either operating system, Mac or Windows) an excellent math font, Noto Sans Math. https://fonts.google.com/noto/specimen/Noto+Sans+Math/glyphs?query=Noto 

Scroll down and expand the glyph sections for Operators and Math Symbols. This is a SIL open-license font provided by Google so you can freely use them in print and digital files. See https://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&id=OFL for licensing information.

You need to provide more information for us to help you: Please tell us what operating system (exactly) you're running. Please tell us (exactly) what InDesign version you're using. Please tell us what font format the Cambria Math fonts are in? (OpenType, TrueType, Type 1)