3+ means 3 and up hope i could help.
Triangles have 3 vertices. The total is 21 vertices. 21 / 3 = 7. He drew 7 triangles.
Use the interior angles theorem for the triangles I hope my answer helped if you have any questions don't be afraid to ask <span>(o^▽^o) <3</span>
If you don't know the derivative of the inverse of sine, you can use implicit differentiation. Apply sine to both sides:

(true for <em>y</em> between -π/2 and π/2)
Now take the derivative of both sides and solve for it:



