Math Destroyer Problem

[Strag]

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Which of the following best represents the value of the expression sq rt of (0.0049 x 1.69)?
a. 0.9
b. 0.09
c. 0.009
d. 0.12
e. 0.012

Answer is b, but how do you approximate this in your head or on board?

 

[Kahr]

Which of the following best represents the value of the expression sq rt of (0.0049 x 1.69)?
a. 0.9
b. 0.09
c. 0.009
d. 0.12
e. 0.012

Answer is b, but how do you approximate this in your head or on board?

First off, you have access to a calculator, so I'd multiply the two numbers right away:
.008281; now I'd convert this to scientific notation, but pick 82.81 x 10^-4, since this number is easier to approximate the sqrt for (rather than 8.281 x 10^-3, and you want the exponent to be an even number so you can simply divide it in half); approximating the sqrt for 82.81 = 9, and 10^-4 becomes 10^-2: 9 x 10^-2 = 0.09

It's often easier to sqrt something in scientific notation, hope that helps.

 

[DDSCaroline]

Here's how you do it in the head. You should see right away that 0.07 is the square root of 0.0049 and 1.3 is the square root of 1.69 (should know 13^2 = 169).

So the problem becomes: sqrt((0.07)^2 * (1.3)^2)
Cancel the sqrts and squares and you get: 0.07*1.3 = 0.091