EM-1525

Writing algebraic expressions

E Math · Secondary 4 · Hard · Writing algebraic expressions

A two-digit number has tens digit

a

and units digit

b

. The number is

10a+b

. The sum of its digits is

a+b

. Show that

(10a+b)-(a+b)

is always a multiple of 9.

It simplifies to $9a$ , which is a multiple of 9.
It simplifies to $a^2+b^2$ , which is a multiple of 9.
It simplifies to $ab$ , which is a multiple of 9.
It simplifies to $10b$ , which is a multiple of 9.

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