Evaluate math expressions via GET or POST as JSON.
MathJS API is a free Science & Math API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
Details from api.mathjs.org.

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The MathJS API is provided by math.js web service (api.mathjs.org).
Yes. The MathJS API is listed in the Science & Math category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The MathJS API requires no authentication, so you can start making requests without signing up.
Yes. The MathJS API supports CORS, so front-end JavaScript can call it directly.
Yes. The MathJS API is served over HTTPS, so it's safe to use in production.