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Multiplying and dividing algebraic fractions

Lessons 32 · 1 lessons · Sh.A. Alimov, O.R. Xolmuhamedov, M.A. Mirzaahmedov. Algebra, Grade 7, revised and expanded 5th edition. O‘qituvchi NMIU, Tashkent, 2017
32

Multiplying and dividing algebraic fractions

Textbook: pp. 144–146
GoalMultiply, divide and raise algebraic fractions to a power.
New words
multiplying fractions · kasrlarni ko‘paytirishreciprocal fraction · teskari kasrdividing fractions · kasrni bo‘lishraising a fraction to a power · kasrni darajaga ko‘tarish
Explanation

When multiplying fractions we multiply the numerators and multiply the denominators: (a/b) · (c/d) = ac/(bd). In division the dividend is multiplied by the reciprocal of the divisor: (a/b) : (c/d) = (a/b) · (d/c) = ad/(bc), with c ≠ 0. A fraction is raised to a power by raising the numerator and denominator separately: (a/b)ⁿ = aⁿ/bⁿ. If we factor the numerators and denominators before multiplying and cancel common factors, the work becomes easier. The result is always written reduced. An expression can be seen as a fraction, a = a/1, so multiplying or dividing a fraction by an expression follows the same rules.

Worked examples
(6x²/(5y)) · (15y²/(2x)) = (6 · 15 · x² · y²)/(5 · 2 · y · x) = 9xy.
((a² - 9)/(a + 2)) : ((a + 3)/(a² - 4)) = ((a - 3)(a + 3)/(a + 2)) · ((a - 2)(a + 2)/(a + 3)) = (a - 3)(a - 2). Power: (2a/(3b²))³ = 8a³/(27b⁶).
Class activity

“Reciprocal cards”: each pupil gets a fraction card and finds its reciprocal. Then in pairs they turn a division problem into multiplication and solve it.

Practice
1
Multiply (4a/(3b)) · (9b²/(2a²)).
2
Perform ((x² - 1)/x) : ((x + 1)/x²).
3
Find the value of 6b/a at a = 2, b = 5.
4
Why is dividing by a fraction the same as multiplying by its reciprocal?