Transcript Estimation

Estimation
Great Marlow School Mathematics Department
Why do we estimate?
• In some circumstances it is unrealistic to
work out the exact answer to a problem, or
you may have to check an answer.
• Therefore an estimate is needed.
• For an estimate, all numbers are rounded
to one significant figure – it makes the
calculation easier.
• Then you do the calculation with the
rounded numbers.
Great Marlow School Mathematics Department
For example…
• 9.7 x 3.1
• Round each number to one significant
figure and you get….
10 x 3 = 30
• This gives you an estimate. The exact or
true answer is:
9.7 x 3.1 = 30.07
Great Marlow School Mathematics Department
Estimate:
• 81.4 x 98.2
• Round each number to 1 significant figure
and you get…
80 x 100 = 8000
• This gives you an estimate.
• The exact or true answer is:
81.4 x 98.2 = 7993.48
Great Marlow School Mathematics Department
Estimate:
• 17.5 x 42
1.5
• Round each number to 1 significant figure
and you get…
20 x 40 = 800 = 400
2
2
• The exact answer is:
490 – check this with your calculator
Great Marlow School Mathematics Department
A more difficult estimate:
• 33.5 + 42.4
0.0237
• Estimate each number to 1 significant
figure:
30 x 40 = 1200 = 60000
0.02
0.02
How did we do this division?
Great Marlow School Mathematics Department
• 1200
0.02
 This division is difficult …. So we have to change the
denominator (the number on the bottom of the
fraction) into a number we can divide by.
• What number could we change it to?
2
• So the calculation would mean making the
denominator 100 times bigger…
• So the numerator (the number on the top) has to be
100 times bigger as well…what you do to the
bottom…you must do to the top!
• The calculation would then be:
 1200 x 100 = 120000 = 60000
0.02 x 100
2
Great Marlow School Mathematics Department