Areas and perimeters
Working out the area or the perimeter of the usual shapes — square, rectangle, triangle, disc — without opening the formula sheet.
Click a card to see its answer. Series no. 821153 — this page’s address gives these exact cards again.
Other series
Arithmetic with signed numbers
Adding, subtracting and multiplying signed numbers, where the signs trip up the reflexes.
Complex numbers
Powers of i, modulus and conjugate: the reflexes that save time on any complex-number calculation.
Decimal form of simple fractions
Going from a simple fraction to its decimal form and back to the fraction, in your head, without long division.
Discriminant and roots
Discriminant, sum and product of the roots of a quadratic, read straight off the coefficients.
Fractions in lowest terms
Reducing a fraction to its lowest terms by spotting the common divisor at a single glance.
GCD and LCM
Finding the greatest common divisor and the least common multiple of two whole numbers, without writing out the factorisation.