Question

(l + b + h)² - 2lb -2bh -2lh..how??

Answer 1

what u want to know i don't understand

Answer 2

Cuboid is made of 6 rectangles. If you consider each rectangle to find area you'll get length breadth or height for particular rectangle. You will add all the areas for curved surface area of rectangle. You'll get two lengths breadths or heights.
Maybe confusing but you'll understand properly when you do practically