• YOU can help the next generation of students in the community!
    Share your trial papers and notes on our Notes & Resources page
  • Like us on facebook here

Another SA of Pyramid Question, please can you check where my mistake/s are: (1 Viewer)

kpad5991

Member
Joined
Dec 29, 2016
Messages
216
Gender
Male
HSC
2018
1630898408528.png
SA front = 1/2 x 12 x 5 = 30 cm^2

SA bottom = 1/2 x 9 x 12 = 54 cm^2

SA back = 1/2 x 15 x 5 = 37.5 cm^2 used Pythagoras to get 15 cm

SARHS = 1/2 x 9 x root 148.75 = 54.9 cm^2 used Pythagoras h^2 = 13^2 - 4.5^2, h = root 148.75

TOTAL SA = 176.4 cm^2

However the answer says 180 cm^2 (what am I doing wrong?)
 

Lith_30

New Member
Joined
Jun 27, 2021
Messages
20
Gender
Male
HSC
2022
You made a mistake in calculating the area of the top triangle. If you look at the pyramid from a birds eye view, the triangle at the very top is a right angle triangle as its vertices bend to the same degree as the triangle at the base.
 

kpad5991

Member
Joined
Dec 29, 2016
Messages
216
Gender
Male
HSC
2018
You made a mistake in calculating the area of the top triangle. If you look at the pyramid from a birds eye view, the triangle at the very top is a right angle triangle as its vertices bend to the same degree as the triangle at the base.
Thank you, but that will still be an SA of 175.5cm^2 the answer says 180 cm^2 (to one decimal place). So not too sure where the 4.5 cm^2 comes from
 

Lith_30

New Member
Joined
Jun 27, 2021
Messages
20
Gender
Male
HSC
2022
Thank you, but that will still be an SA of 175.5cm^2 the answer says 180 cm^2 (to one decimal place). So not too sure where the 4.5 cm^2 comes from
I think I might of explained it badly, the height of the triangle at the top would be the hypotenuse of the triangle at the front. Since they are both right angle triangles.

find the height of the top triangle

the area of the top triangle is 58.5cm^2

Add all the other areas up and the answer should be 180cm^2
 
Last edited:

kpad5991

Member
Joined
Dec 29, 2016
Messages
216
Gender
Male
HSC
2018
I think I might of explained it badly, the height of the triangle at the top would be the hypotenuse of the triangle at the front. Since they are both right angle triangles.

find the height of the top triangle

the area of the top triangle is 58.5cm^2

Add all the other areas up and the answer should be 180cm^2
Thank you
 

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

Top