Đề bài
Dùng định nghĩa hai phân thức bằng nhau chứng tỏ rằng:
\(a)\dfrac{{3{\rm{x}}}}{2} = \dfrac{{15{\rm{x}}y}}{{10y}}\)
\(b)\dfrac{{3{\rm{x}} - 3y}}{{2y - 2{\rm{x}}}} = \dfrac{{ - 3}}{2}\)
\(c)\dfrac{{{x^2} - x + 1}}{x} = \dfrac{{{x^3} + 1}}{{x\left( {x + 1} \right)}}\)
Phương pháp giải - Xem chi tiết
Nếu \(A.D = B.C\) thì \(\dfrac{A}{B} = \dfrac{C}{D}\)
Lời giải chi tiết
a) Ta có:
\(\begin{array}{l}3{\rm{x}}.10y = 30{\rm{xy}}\\{\rm{2}}{\rm{.15x}}y = 30{\rm{x}}y\end{array}\)
Suy ra: \(3{\rm{x}}.10 = 2.15{\rm{x}}y\) nên \(\dfrac{{3{\rm{x}}}}{2} = \dfrac{{15{\rm{x}}y}}{{10y}}\)
b) Ta có:
\(\begin{array}{l}\left( {3{\rm{x}} - 3y} \right).2 = 2.3\left( {x - y} \right) = 6\left( {x - y} \right)\\\left( { - 3} \right).\left( {2y - 2{\rm{x}}} \right) = \left( { - 3} \right).\left( { - 2} \right)\left( {x - y} \right) = 6\left( {x - y} \right)\end{array}\)
Suy ra: \(2.\left( {3{\rm{x}} - 3y} \right) = \left( { - 3} \right).\left( {2y - 2{\rm{x}}} \right)\) nên \(\dfrac{{3{\rm{x}} - 3y}}{{2y - 2{\rm{x}}}} = \dfrac{{ - 3}}{2}\)
c) Ta có: \(\begin{array}{l}\left( {{x^2} - x + 1} \right).x\left( {x + 1} \right) = x.\left( {x + 1} \right)\left( {{x^2} - x + 1} \right) = x.\left( {{x^3} + 1} \right)\\x.\left( {{x^3} + 1} \right)\end{array}\)
Suy ra: \(\left( {{x^2} - x + 1} \right).x.\left( {x + 1} \right) = x.\left( {{x^3} + 1} \right)\) nên \(\dfrac{{{x^2} - x + 1}}{x} = \dfrac{{{x^3} + 1}}{{x\left( {x + 1} \right)}}\)
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