Cho hình thang ABCD(AB//CD) có AB=40cm CD=80cm BC =50cm AD=30cm cmr ABCD là hình thang vuông

From A draw AG // BC cuts CD at WO

We have: Trapezoid ABCD (assumption)

⇒ AB // CD

⇒ AB // GC (because G ∈ CD)

Considering quadrilateral ABCG, there are:

AB // GC (proven above)

AG // BC (assumption)

⇒ Quadrilateral ABCG is a parallelogram

⇒ AB = GC = 40 cm

AG = BC = 50 cm

We have: DG = CD - GC (because G ∈ CD)

DG = 80 - 40

⇒  DG = 40(cm)

Considering ΔAGD, there are:

AG2=AD2+DG2AG2=AD2+DG2

502=302+402⇒502=302+402

⇒502=900+1600⇒502=900+1600

502=2500⇒502=2500

⇒502=502⇒502=502

⇒ AGD square at D

⇒ Trapezoid ABCD is a square trapezoid

the balance is found at the link

step-by-step explanation:

this may not be at all, but i use the same website you're using for school, and if i'm stuck doing one of these i try to look for it on the section.