Write the expression in expanded form (1 + 3b)(6)

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Identity:

An identity is an equality which is true for all values of a variable in the equality.

(a + b)³ = a³+ b³+ 3ab(a + b)

In an identity the right hand side expression is called expanded form of the left hand side expression.

Solution:

(i) (2x + 1)³

Using identity

(a + b)³ = a³+ b³+ 3ab(a + b)

(2x + 1)³

= (2x)³ + 1³ + (3×2x×1)(2x + 1)

= 8x³+ 1 + 6x(2x + 1)

= 8x³ + 1 + 12x² + 6x

(ii) (2a – 3b)³

Using identity,

(a – b)³ = a³–b³ – 3ab(a – b)

(2a – 3b)³ = (2a)³– (3b)³ – (3×2a×3b)(2a – 3b)

=8a³–27b³–18ab(2a –3b)

= 8a³–27b³–36a²b + 54ab²

(iii) [3x/2  + 1]³

Using identity,

(a + b)³ = a³+ b³+ 3ab(a + b)

[3x/2 +1]³

=(3x/2)³+1³+ (3×(3x/2)×1)(3x/2+ 1)

=27x³/8+1+9/2x×(3x/2+1)

= 27x³/8 + 1 + 27/4 x² + 9/2x

= (27/8)x³ + (27/4) x² + 9/2 x + 1

(iv) [x–2/3 y]³

Using identity,

(a - b)³=a³-b³-3ab(a-b)

(X+ 2/3y)³

= (x)³–(2/3 y)³– (3×x×2/3 y)(x – 2/3 y)

= x³– 8y³/27–2xy(x – 2/3 y)

= x³– (8/27)y³–2x²y+ 4/3xy²

Hope this will help you

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