In ICSE Class 9 Mathematics, the topic of trigonometric ratios is introduced, laying the foundation for understanding trigonometry. Here’s a summary of the key points covered in this topic:

1. **Introduction to Trigonometry**: Trigonometry is the study of relationships between the sides and angles of triangles. It is useful in various fields such as physics, engineering, and astronomy.

2. **Trigonometric Ratios**: The three main trigonometric ratios are sine, cosine, and tangent, often abbreviated as sin, cos, and tan, respectively. These ratios are defined based on the sides of a right-angled triangle.

3. **Sine Ratio (sinθ)**: In a right triangle, the sine of an angle is the ratio of the length of the side opposite the angle to the length of the hypotenuse. It is given by sinθ = Opposite/Hypotenuse.

4. **Cosine Ratio (cosθ)**: The cosine of an angle is the ratio of the length of the adjacent side to the length of the hypotenuse. It is given by cosθ = Adjacent/Hypotenuse.

5. **Tangent Ratio (tanθ)**: The tangent of an angle is the ratio of the length of the side opposite the angle to the length of the adjacent side. It is given by tanθ = Opposite/Adjacent.

6. **Reciprocal Trigonometric Ratios**: The reciprocal trigonometric ratios are cosecant (csc), secant (sec), and cotangent (cot), which are the reciprocals of sine, cosine, and tangent, respectively.

7. **Trigonometric Identities**: Various trigonometric identities are used to simplify trigonometric expressions. Some common identities include sin²θ + cos²θ = 1 and tanθ = sinθ/cosθ.

8. **Applications of Trigonometry**: Trigonometry is used to solve problems involving angles and sides of triangles. It is also used in navigation, physics, and engineering for various calculations.

Understanding trigonometric ratios is essential for solving problems involving angles and sides of triangles, as well as for further studies in trigonometry and related fields.