Common Mistakes
The errors students make most, explained one at a time. Each page shows why the mistake happens, the rule done right, a wrong-versus-right example, and a quick check.
Adding Fractions by Adding the Denominators
Adding straight across the top and bottom instead of finding a common denominator first.
MathTreating (a + b) Squared as a Squared Plus b Squared
Squaring each term of a sum separately instead of expanding the whole product.
MathForgetting to Flip the Inequality When Dividing by a Negative
Dividing or multiplying both sides of an inequality by a negative without reversing the sign.
MathDistributing a Negative Sign to Only the First Term
Removing parentheses after a minus sign but only changing the sign of the first term.
MathSquaring a Negative Number Without Parentheses
Reading minus three squared as positive nine, as if the negative were squared too.
MathCancelling a Term Across Addition in a Fraction
Cancelling a number that is added in the top and bottom, instead of a shared factor.
GeometryUsing the Diameter Instead of the Radius in a Circle Formula
Plugging the diameter into a circle formula that calls for the radius.
GeometryMixing Up Square Units and Cubic Units
Labelling an area in cubic units or a volume in square units.
AlgebraConfusing the Slope and the y-Intercept in y = mx + b
Reading the wrong number as the slope in slope-intercept form.
PhysicsThinking Heavier Objects Fall Faster
Assuming a heavier object hits the ground before a lighter one dropped beside it.
PhysicsConfusing Velocity and Acceleration
Assuming large velocity means large acceleration, or that zero velocity means zero acceleration.
PhysicsThinking There Is No Gravity in Space
Believing astronauts float because there is no gravity in orbit.
ChemistryBalancing Equations by Changing Subscripts
Balancing a chemical equation by editing the subscripts inside a formula instead of the coefficients in front.
ChemistryConfusing Atomic Number and Mass Number
Using the mass number to count protons or to identify the element.
ChemistryConfusing Groups and Periods on the Periodic Table
Calling a horizontal row a group or a vertical column a period.
BiologyThinking Plants Do Not Respire
Believing plants only photosynthesize and never carry out respiration.
BiologyConfusing Genotype and Phenotype
Using genotype to mean the visible trait, or phenotype to mean the gene combination.
BiologyConfusing DNA and RNA
Mixing up which nucleic acid is double-stranded and which bases each one uses.
StatisticsConfusing the Median Position with the Median Value
Reporting the middle position number as the median instead of the value found there.
StatisticsReading a Bar Graph as if It Were a Histogram
Treating a bar graph and a histogram as the same kind of chart.
ELAUsing your When You Mean you're
Writing "your" when the sentence means "you are".
ELAConfusing their, there, and they're
Picking the wrong one of their, there, and they're because they sound the same.
ELAConfusing its and it's
Adding an apostrophe to the possessive its, writing it's when its is meant.
ELAConfusing then and than
Swapping the comparison word than with the time word then.
Showing 24 of 150 mistakes