What you need to know
In 25, the base is 2 and the index (also called the power or exponent) is 5. It means 2 × 2 × 2 × 2 × 2 = 32. The index laws are shortcuts that let you simplify expressions without writing out all the multiplications. Every law below only works when the terms have the same base.
| Law | Rule | Number example | Algebra example |
|---|
| Multiplying | am × an = am + n | 34 × 32 = 36 | x5 × x3 = x8 |
| Dividing | am ÷ an = am − n | 57 ÷ 53 = 54 | y9 ÷ y2 = y7 |
| Power of a power | (am)n = am × n | (23)4 = 212 | (k2)5 = k10 |
| Zero index | a0 = 1 (a ≠ 0) | 90 = 1 | (7x)0 = 1 |
| Power of a product | (ab)m = ambm | (2 × 5)3 = 23 × 53 = 1000 | (2x)3 = 8x3 |
Simplify 23 × 24
- Same base (2), so add the indices: 3 + 4 = 7.
- 23 × 24 = 2[sup 7] = 128.
- Check by expanding: 8 × 16 = 128.
Simplify 6x5 ÷ 2x2
- Deal with the numbers (coefficients) first: 6 ÷ 2 = 3.
- Same base x, so subtract the indices: 5 − 2 = 3.
- 6x5 ÷ 2x2 = 3x[sup 3].
Simplify (3a2)3
- The power outside applies to everything inside: 33 and (a2)3.
- 33 = 27. For the power of a power, multiply the indices: 2 × 3 = 6.
- (3a2)3 = 27a[sup 6].
Why is 50 equal to 1?
- Use the division law on 54 ÷ 54: subtract the indices to get 54 − 4 = 50.
- But any number divided by itself is 1: 625 ÷ 625 = 1.
- Both answers must be the same, so 5[sup 0] = 1. The same argument works for any non-zero base.
Terms with more than one letter. Handle the coefficients first, then each base separately. 4m2n × 3mn3: coefficients 4 × 3 = 12; m2 × m = m3 (remember m means m1); n × n3 = n4. Result: 12m3n4.
Common mistake: multiplying the indices when multiplying terms. 23 × 24 = 27, not 212. You only multiply indices for a power of a power, such as (23)4.
Common mistake: applying the laws to different bases. 23 × 32 cannot be simplified with index laws; just evaluate it: 8 × 9 = 72. Also remember that 32 means 3 × 3 = 9, not 3 × 2 = 6.
Check yourself
- Simplify 72 × 75.
- Simplify x9 ÷ x3.
- Simplify (52)3.
- Evaluate 120.
- Simplify 4a3 × 5a2.
- Simplify (2y4)3.
Answers: 1. 77 2. x6 3. 56 4. 1 5. 20a5 6. 8y12