Let a point P inside a parallelogram ABCD be given such that ∠AP B + ∠CP D = 180◦. Prove that

AB · AD = BP · DP + AP · CP (here AB, AD etc. refer to the lengths of the corresponding segments)

Need a pointe as to what methods i should use, just some help would be very nice, im an 11th grade level mathematician so i would understand some concepts but not so much others - again, help on these would be great.

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