2 Math Questions - Area of Circles

Underdog

I'm filling in the homework for my class and I want to make sure I'm doing these two questions correctly.

1. The diameter of a circle is 10 cm. Find the remainin area if a frame of 2cm is to be cut off from the original circle.

Area would have been Pi x 5^2 but when you cut off 2cm for the frame, then the radius is shortened to 3. So I went with:

3^2 x 3.14 = 28.26

2. A circular picture of circumference 88 cm is mounted on a circular cardboard, leaving a margin of 7 cm all round. Find the area of the margin. Use 22/7 as Pi.

So the radius of picture is 88 / 22/7 = 14.

The radius of the whole cardboard then is 14 + 7 = 21.

So the area of the margin is:

21^2 x 22/7 - 14^2 x 22/7 = 770

Thanks!

wmelon

the first one is correct.

the second one 14 cm is the diameter not the radius.

Underdog

Oh whoops, I forgot to divide by 2. Ok so then it'll be

14^2 x 22/7 - 7^2 x 22/7 = 462

localh77

Underdog wrote: »

So the radius of picture is 88 / 22/7 = 14.

I think your answer was right the first time, you just wrote this down wrong. It should be


radius of picture = 88 / 22/7 / 2 = 14.

Underdog

... oh poppycock. Yeah, I was copying my work off the sheet and missed the divide by 2. So it should still be

21^2 x 22/7 - 14^2 x 22/7 = 770

tarnok

I didn't check the actual arithmetic but the way you've set it up looks correct.