Diagonal of Parallelogram Formula

Diagonal of Parallelogram Formula

A parallelogram is a quadrilateral whose opposite sides are parallel and equal. The opposite sides being parallel and equal, forms equal angles on the opposite sides. Diagonals of a parallelogram are the segments which connect the opposite corners of the figure.

 Diagonal of a Parallelogram

p,q are the diagonals 

a,b are the parallel sides

\[\LARGE p=\sqrt{a^{2}+b^{2}-2ab\cos (A)}=\sqrt{a^{2}+b^{2}+2ab\cos (B)}\]

\[\LARGE q=\sqrt{a^{2}+b^{2}+2ab\cos (A)}=\sqrt{a^{2}+b^{2}-2ab\cos (B)}\]

\[\LARGE p^{2}+q^{2}=2(a^{2}+b^{2})\]

Solved Examples

Question 1:

Find the diagonal of a parallelogram with sides 3 cm, 5 cm and angle 45 degrees ?


Given a = 3 cm
b = 5 cm
angle A = 45°
Formula of diagonal is,

q =

\(\begin{array}{l}\sqrt{a^{2}+b^2-2ab cosA}\end{array} \)

q = 

\(\begin{array}{l}\sqrt{3^{2} + 5^2 – 2\times 3 \times  5 cos 45}\end{array} \)
q =
\(\begin{array}{l}\sqrt{34 – 30\times 0.707 }\end{array} \)

q = √12.79

=3.576 cm

Diagonal  of parallelogram = 3.576 cm.


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  1. simple and good job.