Answer:
B
Explanation:
"lateral" means "belonging to the side".
so, the lateral area of a square pyramid (or any kind of pyramid or other 3-D structure, really) is the sum of the areas of its side faces.
so, in essence we have to calculate the area of one of the side triangles, and then multiply it by 4, as the other 3 are equal (due to the fact that the base is a square, and we are clearly dealing with a regular, symmetric pyramid).
each of the side triangles is an isoceles triangle (both legs are equal long, therefore both side angles are equal too).
that means both side angles are 60° each.
and because we know that the sum of all angles in a tingle is always 180°, the angle at the top is
180 - 60 - 60 = 60°
hey, all 3 angles are the same !
that means the triangle is not only isoceles but equilateral (all 3 sides are equally long).
and that means all sides are 18 cm long.
the area of a triangle can be calculated for our scenario in 2 major ways :
1. baseline × height / 2
2. Heron's formula (since we know all 3 sides) :
A = sqrt(s(s-a)(s-b)(s-c)), s = (a+b+c)/2
I don't know which one your teacher wants you to use.
so,
1.
the height we get via Pythagoras
leg² = (baseline/2)² + height²
18² = (18/2)² + height²
324 = 81 + height²
81×4 = 81 + height²
81×3 = height²
height = sqrt(81×3) = 9×sqrt(3)
the area of the triangle is
18 × 9 × sqrt(3) / 2 = 9 × 9 × sqrt(3) = 81×sqrt(3) cm²
2.
s = (18+18+18)/2 = 54/2 = 27
A = sqrt(27(27-18)(27-18)(27-18)) =
= sqrt(9×3×9×9×9) = 3×3×3×3×sqrt(3) = 81×sqrt(3) cm²
the total lateral area is then
4×81×sqrt(3) = 324×sqrt(3) = 561.1844617... cm²
oh, I see, i think you did not include the square root in the answer options. that is confusing.
but as you can see, option B must be correct.