Answer:
The values of sin θ and cos θ represent the legs of a right triangle with a hypotenuse of 1; therefore, solving for cos θ finds the unknown leg, and then all other trigonometric values can be found.
As you increase the subintervals the area will be closer and closer to the real value. In other words your approximation gets better.
As you increase the intervals, there will be more rectanagles and the added area of these rectangles are converging towards the actual area under the curve.
