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which of the following expressions is 0.00012 written in scientific notation?

1.2 × 10 -4

.12 × 10 -3

1.2 × 10 4

12 × 10 -5

1.2 x 10^-4

Scientific notation is basically a number (that must be between 1 and 10) multiplied by a power of 10. Notice to get a number between 0 and 10, you must move the decimal point 4 places to the right to obtain 1.4; therefore, your power of 10 is 10^-4.

Okay, so if you're trying to figure something like this out quickly, look at the exponent number, and see if it's positive or negative. If it's positive, move the decimal in your number to the right. If it's negative, move the decimal in your number to the left. Easy trick my science teacher taught me :)
In this case, it's going to be .12 x 10 -3 because if you do that decimal trick, you move the decimal three places. .00012