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Substitution

Substitution means putting numbers in place of letters to calculate the value of an .

For example, in the expression \(2b^2c\), where \(b = 4\) and \(c = 3\), use the values of \(b\) and \(c\) to calculate the numerical value of the expression:

\(2b^2c = 2 \times b^2 \times c\)

Remember that the rules of BIDMAS/BODMAS show that the order of operations (the order sums should always be completed in) is: Brackets, Indices or Powers, Divide/Multiply and Add/Subtract. This means that the value of \(b^2\) should be calculated before multiplying by 2 or \(c\), as indices come before multiplication.

This gives: \(2b^2c = 2 \times b^2 \times c = 2 \times 4^2 \times 3\) (substituting \(b = 4\) and \(c = 3\)) = \(2 \times 16 \times 3 = 96\)

Question

Work out the value of \(d + (3e + f)^2\) when \(d = 2\), \(e = -3\) and \(f = 1\).

Substitution into formula

Substitution into formulae works the same way as substitution into expressions, and it is important to follow the rules of BIDMAS.

A common formula is used to convert Fahrenheit (F) to Celsius (C): \(F = \frac{9C}{5} + 32\)

Example

What is the temperature in Fahrenheit if it is 20 °C?

Substitute the value of \(C\) into the equation \(F = \frac{9C}{5} + 32\) to work out the temperature in Fahrenheit.

\(F = \frac{9C}{5} + 32 = \frac{9 \times 20}{5} + 32 = \frac{180}{5} + 32 = 36 + 32 = 68^\circ \text{F}\)

The temperature of 20 °C is the same as 68 °F using the formula.

Question

The formula to work out the force in Newtons of an object is \(F = ma\), where \(m\) is mass in kilograms and \(a\) is the acceleration of the object. What is the force of an object that has a mass of 16 kg and an acceleration of 7 m/s2?