Question

About how far does the tip of a 1.8m pendulum rise as it swings 24 degree to one side?

I use the cosine to find how far the tip of pendulum rise. i get 1.9m.

Answer 1

Consider your answer. The angle traverse is quite small. Will it really rise more than its entire length?

Draw a diagram. after swinging through an angle θ, the distance from the top is 1.8cosθ.

So, the amount it has risen is 1.8(1-cosθ)

I suspect you used r(1+cosθ).