2. Calcule sen 2a, dada la condición siguiente. Sugerencia: utilice las identidades pitagóricas y la figura del plano de los
cuadrantes.
Si sen a= 4/5 y a está el cuadrante II.
Calcule sen (a/2) y cos (a/2), a partir de la información dada.
sen a = - 2/3 y se sabe que a está en el cuadrante IV.

what statement? is there an image?

jbankston is right, use thhe link and you should find the answer.

alright so i think for this one it would to draw it out. this may be hard for me to try and draw, but i will do my best on here:

        r

  /       |     \

notice how it forms 2 triangles inside one bigger triangle.

so if we know the angle spr is the same as psr that is saying the measure of angle p is the same as angle s. this means the big triangle psr is either isosceles or equilateral. in both cases, the triangle is perfectly symmetrical when split down the middle, which is in this case rq. because we know it is perfectly symmetrical on both sides, we know that the two triangles inside the bigger one are equal.