[Guest]

Given: Use parentheses if necessary to prove that the following expression is 21.

2 * 5 - 1/2 * 10 * 9 = 21

First, 2(5) = 10 - (.5)(10)9 = -35

2nd, 2(5-1/2*10)9 = 2(0)(9) = 0

3rd, 2 (5-1/2) 10 (9) = 810.

I am stuck I can't prove that the expression above is 21.

[Thalia]

Does the original problem use the * or the multiplication x? I don't think it makes a difference but just in case. . .

Edited February 6, 2011 by Thalia

[Guest]

Does the original problem use the * or the multiplication x?

Hi Thalia, I double check the book and it shows only the asterisk sign *.

[Thalia]

Well, good luck. Unless there is an alternative meaning for the * or you can add other symbols, it doesn't look like it's going to work. One thing you could experiment with is putting parentheses between the numbers. 2(1) instead of 21 so it equals 2. Another thing to consider is replacing the / with the division sign (the line with 2 dots). I'm still not getting anything from that but maybe you will.

[curr3nt]

I believe this is impossible without some trick.

The * 9 doesn't work because then 2 * 5 - 1/2 * 10 = 21/9 = 7/3 there is no way to get a /3.

Using parentheses to get the * 9 in the division would also get the * 10 and there is no way to get a large enough number on top.

[Guest]

There are no ways to prove what you need. The problem itself is WRONG !