Discuss and discover Working with a classmate, copy and complete the table below. 4
_ 4
_ 2
2 × 2
_ 6
6 × 6 × 6
_ y
_ a2
a3
_ b2
b5
_ x3
x6 y × y × y
Written as a power of 4 = Written as a power of 2 = Written as a power of 6 = Written as a power of y = Written as a power of a = Written as a power of b = Written as a power of x =
4 0 = 1 2 6 2
Look carefully at the table to see if you can discover a rule (using powers) to fi nd the missing answers.
What have you learned about dividing terms and powers from completing this table?
Law 2: Dividing numbers in index form Consider dividing a7 a 7 ÷ a3 = a7
by a3
_ a3
. = a × a × a × a × a × a × a
________________ a × a × a
Three of the as at the top and the three as at the bottom can be divided out: = (a × a × a)1 × a × a × a × a
__________________ (a × a × a) 1
So, we are now left with a4
__ 1 , or simply a4
The same answer is obtained by subtracting the indices, that is, 7 − 3 = 4 . This suggests our second law.
Law 2: The second law of indices tells us that when dividing a number with an exponent by the same number with an exponent, we have to subtract the exponents (powers). ap
_ aq
= ap − q The formula appears in the formulae and tables booklet.
Worked example Simplify the following, leaving your answer in index form. (i) 56
__ 54
Solution (ii) (ii) x10
___ x7
By the end of this section you should be able to: ● divide variables with powers