ABCD is a parallelogram with ∠ABC = 90∘, then number of right angles in the parallelogram excluding ∠ABC is/are ____?
3
ABCD is the given parallelogram.
In a parallelogram, opposite internal angles are equal.
So ∠ABC = ∠ADC=90∘
Also, ∠BAD = ∠BCD.
In a quadrilateral, sum of all internal angles is 360∘.
So ∠BAD + ∠BCD = 180∘.
Therefore ∠BAD = ∠BCD = 90∘.
So the number of internal right angles is 4 and excluding ∠ABC it will be 3.