Standard form The teaching page behind this test: the rule, worked examples set out as a mark scheme, and the four mistakes that lose most of the marks. Index laws Practice. The rule for adding and subtracting powers, on its own and away from standard form. Surds Practice. Higher tier, and the next subtopic along: simplifying, and rationalising a denominator. All Number topics Back to the Number menu, with every topic and practice page in this part of the course.