Evaluating Expressions by Substitution

Substitution into Expressions — Examples

If a = 2, b = 3 and c = −4, find the value of:

  1. a + b
  2. 2a − b
  3. 3a2 − 2c
  4. 2c2 − 4ab

Solutions

  1. a + b = 2 + 3 = 5
  2. 2a − b = 2(2) − 3 = 4 − 3 = 1
  3. 3a2 − 2c = 3(22) − 2(−4) = 3(4) + 8 = 20
  4. 2c2 − 4ab = 2(−4)2 − 4(2)(3) = 2(16) − 24 = 32 − 24 = 8

If x = 3 and y = −2, find the value of 3x2 − 4xy − 2y2.

Solution

3x2 − 4xy − 2y2 = 3(32) − 4(3)(−2) − 2(−2)2 = 3(9) + 24 − 2(4) = 27 + 24 − 8 = 43.

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Substituting — Polypad
Junior Cycle • Algebra

If \( x = 2 \), explain why \( x^2 + 3x - 4 = 6 \).

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