Substituting into Formulae

In algebra, substitution is putting numbers in place of letters to calculate the value of an expression. For example:

If e=3 and f=4. what is that value of 40ef?

40ef is the same as writing 40 multiplied by e multiplied by f.

First of all, as we know that e=3, we replace e with 3 in the expression and we are left with: 40 multiplied by 3 multiplied by f.

Likewise, we do the same with f. We replace f with 4 in the expression and we are left with: 40 multiplied by 3 multiplied by 4.

Therefore, 40ef=40\times 3\times 4=280.

A formula is a mathematical relationship connecting two or more quantities. For example, F = ma, expressing F in terms of m and a.

X=\frac { -b+ \sqrt { { b }^{ 2 }-4ac } }{ 2a } is a formula giving x in terms of the quantities of a, b and c.

From F=ma, we can find the value of F if we are given values for m and a, which you can see in the example below.

Example

Find the value of F if m = 3 and a = 14

F = ma

= 3 \times 14

= 42


Example

From the formula X=\frac { -b+ \sqrt { { b }^{ 2 }-4ac } }{ 2a }, if we are given that a = 2, b = 5 and c = −12, find the value of x.

We know that a=2, b=5 and c=−12, so:

X=\frac { -5+ \sqrt { { 5 }^{ 2 }-4(2)(-12) } }{ 2(2) }

X=\frac { -5+\sqrt { 25+96 } }{ 4 }

=\frac { -5+\sqrt { 121 } }{ 4 }

=\frac { -5+11 }{ 4 }

=\frac { 6 }{ 4 }

=\frac { 3 }{ 2 }


Example

Find the value of T when m = 5, g = 10 and w = -3, that:

T = mg + mw(g^{ 2 } - 81).

T = mg + mw({ g }^{ 2 } - 81)

Substituting m = 5, g = 10 and w = -3 gives:

5(10) + 5(-3) (10^{ 2 } - 81)

= 50 - 15(100 - 81)

= 50 - 15(19)

= 50 - 285

= -235


Example

Find the value of S if:

S = ut + \dfrac { 1 }{ 2 }{ at }^{ 2 }</mark>

When u = 3, t = 5 and a = 4.

S = ut + \dfrac { 1 }{ 2 }{ at }^{ 2 }

= (3) (5) + \dfrac{ 1 }{ 2 } (4) (5^{ 2 })

= 15 + 50

= 65


Example

Given that { V }^{ 2 } = u^{ 2 } + 2aS. Find the possible values of V when u = 4, a = 5 and S = 4.8

Substituting u = 4, a = 5 and S = 4.8 into the formula gives us:

V^{ 2 } = 4^{ 2 } + 2(5) (4.8)

{ V }^{ 2 } = 16 + 10 \times 4.8

{ V }^{ 2 } = 16 + 48

{ V }^{ 2 } = 64

V = \sqrt { 64 }

V = -8 or +8 = \pm8.


Example

Find the possible values of x if x = \sqrt {a^{ 2 }+b^{ 2 } }, when a = 5 and b = 12

So, we will use the formula x = \sqrt { { a }^{ 2 }+{ b }^{ 2 } }

Substituting a = 5 and b = 12 gives us:

x = \sqrt { { 5 }^{ 2 }+{ 12 }^{ 2 } }

= \sqrt { 25+144 }

= \sqrt { 169 }

= \pm13


Example

Find the value of w if w = { u }^{ 2 }+{ v }^{ 2 }+{ z }^{ 2 }+{ a }^{ 3 }+{ b }^{ 3 } if:

u = 1, v = 2, z = 3, a = 0, b=4.

We substitute u=1, v=-2, z=-3, a=0 and b=4 into the formula to get:

w = { 1 }^{ 2 }+(-2)^{ 2 }+{-3}^{ 2 }+{ 0 }^{ 3 }+{ 4 }^{ 3 }

= 1 + 4 + 9 + 0 + 64

= 78


Example

Evaluate y in each of the following expressions, given that:

x = 2, z = 3 and w = -1

i) y = { x }^{ z } + { z }^{ x } - w

ii) y = { z }^{ x } + w - { w }^{ 2 }

iii) y = \frac { { x }^{ 2 }-{ w }^{ 2 } }{ z }

iv) y = \frac { x+z+w }{ xzw }

v) y = \frac { xzw }{ x+z+w }

i) y = { x }^{ z } + { z }^{ x } - w

= { 2 }^{ 3 } + { 3 }^{ 2 } - (-1)

= 8 + 9 + 1

= 18

ii) y = { z }^{ x } + w - { w }^{ 2 }

= { 3 }^{ 2 } + (-1) - { (-1) }^{ 2 }

= 9 - 1 - 1

= 7

iii) y = \frac { { x }^{ z }-{ w }^{ z } }{ z }

= \frac { { 2 }^{ 2 }-{ (-1) }^{ 2 } }{ 3 }

=\frac{4-1}{3}

= \frac{3}{3}

=1

iv) y = \frac{x+z+w}{xzw}

= \frac{2+3-1}{2(3)(-1)}

= \frac{4}{-6}

= \frac{-2}{3}

v) y = \frac {xzw}{x+z+w}

*** QuickLaTeX cannot compile formula:
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*** Error message:
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leading text: $<span style="color: #

*** QuickLaTeX cannot compile formula:
<span style="color: #000000;">= \frac{-6}{4}

*** Error message:
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= \frac{-3}{2}


Example

Find the value of w when:

x = -2, y = -3, x = \frac { 1 }{ 4 } and w = { (\frac { xy }{ z } +3) }^{ \frac { 1 }{ 3 } }

Substituting x = -2, y = -3 and z = \frac { 1 }{ 4 } into the formula gives:

w = { (\frac { (-2) (-3) }{1/4} +3) }^{ \frac { 1 }{ 3 } }

= { (\frac { 6 }{ 1/4 } +3) }^{ \frac { 1 }{ 3 } }

= { (24+3) }^{ \frac { 1 }{ 3 } }

= { 27 }^{ \frac { 1 }{ 3 } }

= { { (3 }^{ 3 }) }^{ \frac { 1 }{ 3 } }

= 3


Example

Giving that E = { mc }^{ 2 }. Find the value of E when m = 100,000,000 and c = 300,000,000, giving an answer in standard form.

Substituting m = { 10 }^{ 8 }, c = 3 \times { 10 }^{ 8 } in E = { mc }^{ 2 } gives:

E = { 10 }^{ 8 } \times { (3\times { 10 }^{ 8 }) }^{ 2 }

*** QuickLaTeX cannot compile formula:
= { 10 }^{ 8 } \times { 3 }^{ 2 } \times ({ 10 }^{ 8 }) }^{ 2 }

*** Error message:
Extra }, or forgotten $.
leading text: ... \times { 3 }^{ 2 } \times ({ 10 }^{ 8 }) }

= { 10 }^{ 8 } \times { 3 }^{ 2 } \times { 10 }^{ 16 }

= { 3 }^{ 2 } \times { 10 }^{ 8 + 16 }

= 9 \times { 10 }^{ 24 }


Example

Find the value of x if y=11115 and w=294, given that x = y^{2} - w^{2}

We substitute y=11115 and w=294 into the formula, which gives us:

x = 11115^{2} - 294^{2}

= (11115+294)(11115-294)

=11409 \times 10821

= 123,456,789