the cover of a textbook is 9x11 inches. What is the ratio of its base to its height? Does the cover of the textbook represent a golden triangle?

Hello! By "golden triangle," you likely meant "golden rectangle."

The length of a golden rectangle is appox 1.618 the width.

On the other hand, the dimensions 9 by 11 boil down to 1.222; this means that the length is approx. 1.222 times the width.

No, the textbook does not have the proportions of a golden rectangle.