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Competition - post a photo you've taken of nature >>

Surface area

"A picture frame is made by cutting out a 7×10×1 rectangular prism from the center of a 9 × 12 × 1 rectangular prism. What is the surface area of the picture frame?"

Any ideas? I tried 2(7*10+10*1+1*7)-2(9*12+12*1+1*9) = -84 but obviously that can't be right.
(edited 5 years ago)
Reply 1
Avatar for JSG29
JSG29
11
I'd suggest drawing the shape you need to find the surface area of (easy enough in 2D, if you can't draw it in 3D) - should make it easier.
Consider the top and bottom of the new shape, then consider the outer sides and inner sides.

Top & Bottom:

Spoiler


Outer Sides:

Spoiler


Inner Sides:

Spoiler

(edited 5 years ago)
Reply 2
Avatar for Prasiortle
Prasiortle
OP
13
Original post by JSG29
I'd suggest drawing the shape you need to find the surface area of (easy enough in 2D, if you can't draw it in 3D) - should make it easier.
Consider the top and bottom of the new shape, then consider the outer sides and inner sides.

Top & Bottom:

Spoiler


Outer Sides:

Spoiler


Inner Sides:

Spoiler




Thanks for your assistance - much appreciated.
Reply 3
Avatar for Ryanzmw
Ryanzmw
16
Definitely a question which is looking to catch out everyone who didn't draw a picture :h:

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