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A4. Real numbers : standard index form. First remember that: 10=101 100=102 1000=103 10000=104 0.1=10-1 0.01=10-2 0.001=10-3 0.0001=10-4 Now all numbers can be written in a form that uses powers of 10. 200=2 × 102 3000=3 × 103 0.6=6 × 10-1 312=3.12 × 102 85=8.5 × 101 0.0008=8 × 10-4 0.04=4 × 10-2 334682=3.34682 × 105   When a number is written in this form, i.e., in the form a x 10b, where 1 £ a < 10, then it is said to be in standard index form (also known as standard form or scientific notation). Very large or very small numbers can be written this way. For example, 1 billion=1000000000=1 × 109 You should be able to convert to standard index form to make sensible estimates for calculations involving multiplication and/or division. Convert any number to standard index form and check your answer. Press "Put in standard form" to get a number chosen at random, or enter your own number. If your answer is wrong, then help is given so you can see where your mistake is. = × 10