# Help with perimeter ( math mammoth)

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The two sides of a right triangle that are perpendicular to each other, measure 5cm and 12 cm . What is the perimeter?

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Math mammoth 4b

How do you get that?

Thanks!

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Okay, I haven't had geometry for a long time, but...

A squared + B squared = C squared for triangles

So (5x5) + (12x12) = 169

Square root of 169 = 13

So, 13+12+5 = 30

Right???? :lol:

ETA: But I guess that's not a 4th grade way to solve it...maybe they are supposed to draw a picture with a ruler to get the answer?

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Okay, I haven't had geometry for a long time, but...

A squared + B squared = C squared for triangles

So (5x5) + (12x12) = 169

Square root of 169 = 13

So, 13+12+5 = 30

Right???? :lol:

ETA: But I guess that's not a 4th grade way to solve it...maybe they are supposed to draw a picture with a ruler to get the answer?

that's correct. I cannot think of a way to solve this without Pythagorus (other than to just know that 5,12,13 is one of the 'special' right triangles where all the sides are an integers). There are geometric ways to derive pythagorus that might work...

http://w3.uwyo.edu/~lane/

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I looked at the question in my dd's 4B and it appears that the rest of the directions are to "Draw the triangle and find the perimeter," so, I'm guessing she just wants you to draw it and measure the 3rd side. That side should measure 13. Then, adding the three sides would give you 30.

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No,it says perimeter ,not area, which would have made sense. It's strange because in MM 4 Pythagoras wasn't introduced yet,so I'm not sure if there would be another way to solve it .

Area will be introduced in next chapter ,so I think the author meant perimeter.

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I looked at the question in my dd's 4B and it appears that the rest of the directions are to "Draw the triangle and find the perimeter," so, I'm guessing she just wants you to draw it and measure the 3rd side. That side should measure 13. Then, adding the three sides would give you 30.

But she doesn't give the total perimeter (30), so I don't understand how do you get 13?

She only gives the two sides and then asks to find the perimeter.

I am using MM randomly as a supplement to Singapore so maybe I am missing something...

Edited by blessedmom3
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But she doesn't give you the total perimeter (30), so I don't understand how do you get 13?

She only gives you the two sides and then asks to find the perimeter.

I am using MM randomly as a supplement to Singapore so maybe I am missing something...

If you draw the two known sides with the correct length at right angles to each other, you can then connect the open ends to make the third side.

Though I'd probably go ahead and teach Pythagoras, just for fun. :D

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I looked at the question in my dd's 4B and it appears that the rest of the directions are to "Draw the triangle and find the perimeter," so, I'm guessing she just wants you to draw it and measure the 3rd side. That side should measure 13. Then, adding the three sides would give you 30.

:iagree: This came after using a protractor to draw angles so I think they are supposed to measure out two known sides and the angle, then connect to get the third side.

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If you draw the two known sides with the correct length at right angles to each other, you can then connect the open ends to make the third side.

Though I'd probably go ahead and teach Pythagoras, just for fun. :D

Would you explain how do you draw the right angles without a protractor? I just ordered one :) I assumed you don't need one for 4th grade.

He tried drawing,but he never got 13 for the other side.

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Would you explain how do you draw the right angles without a protractor? I just ordered one :) I assumed you don't need one for 4th grade.

He tried drawing,but he never got 13 for the other side.

Do you have a piece of card stock? You can trace the corner.

I've used a ruler, but it is hard. I had to buy a protractor for 4th grade math when we were there. :tongue_smilie:

I wouldn't let this problem be a show stopper for my kid. Just tell him it's 13, explain Pythagoras (I think I showed this AoPS video to my son around that time, though it uses some algebra). Let him find the perimeter. My son had a hard time with anything requiring him to draw geometric items, as he was in 2nd grade at the time. So I just took anything that looked close enough. Even with a protractor, he wasn't exact. His fine motor skills aren't there to be such. As long as he understood the concepts, I was happy. :)

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Or let the corner of a piece of paper be your right angle. Mark 12 cm on one side, 5 cm on the other.

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Would you explain how do you draw the right angles without a protractor? I just ordered one :) I assumed you don't need one for 4th grade.

He tried drawing,but he never got 13 for the other side.

Draw it on 1/4" graph paper, 4 squares per unit, then measure the diagonal side with a ruler. Use two rulers butted onto each other to form the square. Buy a set square. Any of those will work, and a set square is very useful for parallel lines later.

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Would you explain how do you draw the right angles without a protractor?

You can either "steal" a right angle by using a geometry triangle (or a rectangular piece of cardboard) - or easiest: draw on graph paper.

To construct a right angle with compass and ruler:

Draw a line. We will erect a perpendicular bisector in the middle:

Take a protractor. Using a radius larger than half the line, draw a half circle with the center at one end. Carefully preserving the radius, draw a half circle with the center at the other end of the line. The circles intersect once above and once below your line. Connect these two points - the line you have constructed will be perpendicular to your original line.

Now you can measure your desired lengths along each of these lines.

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Ok, I'm so slow to get it!

Finally after drawing it, we got it, buthe loved Pythagoras better :):)

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