Shortcut Trick
In an isosceles trapezium, the projection of the non-parallel side on the base is given by: projection = (BC − AD) ÷ 2 = (14 − 8) ÷ 2 = 3 cm.
Using the Pythagorean triplet (3, 4, 5) for the right-angled triangle formed by the height, projection, and slant side (5 cm), the height (h) is directly found to be 4 cm.
Area of the trapezium = Average of parallel sides × height = ((8 + 14) ÷ 2) × 4 = 11 × 4 = 44 cm2.
∴ The correct answer is 44 cm2.
Alternate Method
Given:
Parallel sides: AD = 8 cm, BC = 14 cm
Non-parallel sides: AB = CD = 5 cm
Formula Used:
Area of trapezium = (1 ÷ 2) × (sum of parallel sides) × height
By Pythagoras theorem: Hypotenuse2 = Base2 + Height2
Calculations:
Draw perpendiculars AP and DQ from A and D to BC respectively.
⇒ PQ = AD = 8 cm
⇒ BP = QC = (BC − PQ) ÷ 2
⇒ BP = (14 − 8) ÷ 2 = 3 cm
In right-angled triangle APB:
⇒ AB2 = AP2 + BP2
⇒ 52 = AP2 + 32
⇒ 25 = AP2 + 9
⇒ AP2 = 16
⇒ AP = 4 cm (height, h)
Now, calculate the area of the trapezium ABCD:
⇒ Area = (1 ÷ 2) × (AD + BC) × AP
⇒ Area = (1 ÷ 2) × (8 + 14) × 4
⇒ Area = (1 ÷ 2) × 22 × 4
⇒ Area = 11 × 4 = 44 cm2
∴ The correct answer is 44 cm2.
Additional Information
Isosceles Trapezium Properties
The non-parallel sides are equal in length (AB = CD), the base angles are equal (∠B = ∠C), and the diagonals are equal in length (AC = BD).
Pythagorean Triplets
Common right-angled triangle side ratios like 3 : 4 : 5, 5 : 12 : 13, and 8 : 15 : 17 help find heights or slant sides instantly without full calculation.
Cyclic Quadrilateral Connection
Every isosceles trapezium is a cyclic quadrilateral, meaning all its vertices lie on a single circle, and opposite angles sum to 180°.