DIFFERENCE BETWEEN EQUATION,EXPRESSION, IDENTITY AND FORMULAE In Algebra, you use letter symbols to represent unknowns in a variety of situations: EQUATIONS

EXPRESSIONS

IDENTITIES

FORMULAE

In an equation the letters stand for one or more particular numbers (the solutions of the equation). For example: 2x + 1 = x – 2 In an expression there is no equals sign. For example: 3x² + 2x – 1 In an identity there is an equals sign, but the equality holds for all values of the unknown. For example: 2(x + 1) = 2x + 2 In a formula, letters stand for defined quantities or variables. For example: d = s · t (d is distance, s is speed and t is time)

Exercise. Separate the equations, the formulae, the identities and the expressions: a) x ( x + 1 ) = x² + x

c) V = I · R

e) 7x + 11 = x – 9

g) A = π · r²

b) 7y + 10

d) x² - 3x + 10

f) (x + 1)² = x² + 2x + 1

h) x² - 7x = 0