Calculate the following without the use of calculators.
a. \((-3x^3y^2)(2x^6y^5)\)
b. \((2b)^{-4}\)
c. \(\left(\dfrac{-6x^6}{3x^3}\right)^{-2}\)d. \(81^{\frac{3}{4}}\)
e. \(\dfrac{-12m^4n^3(m^3n^2)}{36m^7n^5}\)
Solution:
a. For the expression,
\[(-3x^3y^2)(2x^6y^5)\]
We express them as separate products,
\[(-3x^3y^2)(2x^6y^5)=(-3\times x^3\times y^2)\times (2\times x^6\times y^5)\]
We expand the brackets,
\[(-3x^3y^2)(2x^6y^5)=-3\times x^3\times y^2\times 2\times x^6\times y^5\]
Next, we bring like terms together,
\[\begin{align}(-3x^3y^2)(2x^6y^5)&=-3\times 2\times x^3\times x^6\times y^2\times y^5=\\&=-6\times \left(x^{3+6}\right)\times\left(y^{2+5}\right)=\\&=-6\times x^9\times y^7=\\&=-6x^9y^7\end{align}\]
b. For the expression,
\[(2b)^{-4}\]
We first get rid of the negative exponent, we apply the reciprocal rule,
\[(2b)^{-4}=\dfrac{1}{(2b)^4}=\dfrac{1}{2^4b^4}=\dfrac{1}{16b^4}\]c. For the expression,
\[\left(\dfrac{-6x^6}{3x^3}\right)^{-2}\]
To get rid of the negative exponent, we apply the reciprocal rule,
\[\left(\dfrac{-6x^6}{3x^3}\right)^{-2}=\left(\dfrac{3x^3}{-6x^6}\right)^2\]
We then divide the like terms of the expression in the bracket,
\[\left(\dfrac{-6x^6}{3x^3}\right)^{-2}=\left(\dfrac{3x^3}{-6x^6}\right)^2=\left(-\dfrac{1}{2}\left(x^{3-6}\right)\right)^2=\left(-\dfrac{1}{2}x^{-3}\right)^2\]
Afterward, we distribute the exponent \(2\) to the product inside the bracket to get,
\[\begin{align}\left(\dfrac{-6x^6}{3x^3}\right)^{-2}=\left(-\dfrac{1}{2}\times \dfrac{1}{x^3}\right)^2=\\&=\left(-\dfrac{1}{2x^3}\right)^2=\\&=\dfrac{(-1)^2}{(2x^3)^2}=\\&=\dfrac{1}{2^2(x^3)^2}=\\&=\dfrac{1}{4x^6}\end{align}\]
d. For the expression,
\[81^{\frac{3}{4}}\]
we recall first the fraction exponent rule,
\[81^{\frac{3}{4}}=\left(\sqrt[4]{81}\right)^3=\sqrt[4]{81^3}\]
But
\[81=9^2=\left(3^2\right)^2=3^4\]
Hence
\[81^{\frac{3}{4}}=\sqrt[4]{81^3}=\sqrt[4]{\left(3^4\right)^3}=\sqrt[4]{\left(3^3\right)^4}=3^3\]
We can see in another way, we recall that,
\[\left(a^m\right)=a^{mn}\]
Thus,
\[81^{\frac{3}{4}}=\left(3^4\right)^{\frac{3}{4}}=3^{4\times \frac{3}{4}}=3^3\]
e. For the expression,
\[\dfrac{-12m^4n^3(m^3n^2)}{36m^7n^5}\]
We first expand the numerator,
\[\dfrac{-12m^4n^3(m^3n^2)}{36m^7n^5}=\dfrac{-12\times m^4\times n^3\times m^3\times n^2}{36m^7n^5}\]
We next bring like terms together, to get
\[\begin{align}\dfrac{-12m^4n^3(m^3n^2)}{36m^7n^5}&=\dfrac{-12\times m^4\times m^3\times n^3\times n^2}{36m^7n^5}=\\&=\dfrac{-12\times m^{4+3}\times n^{3+2}}{36m^7n^5}=\\&=\dfrac{-12\times m^7\times n^5}{36m^7n^5}=\\&=\dfrac{-12\times m^7\times n^5}{36\times m^7\times n^5}\end{align}\]
We then divide like terms to get,
\[\begin{align}\dfrac{-12m^4n^3(m^3n^2)}{36m^7n^5}&=\left(\dfrac{-12}{36}\right)\times \left(\dfrac{m^7}{m^7}\right)\times\left(\dfrac{n^5}{n^5}\right)=\\&=\left(-\dfrac{1}{3}\right)\times m^{7-7}\times n^{5-5}=\\&=\left(-\dfrac{1}{3}\right)\times m^0\times n^0\end{align}\]
We recall that any nonzero number raised to the exponent \(0\) is \(1\), we get
\[\dfrac{-12m^4n^3(m^3n^2)}{36m^7n^5}=\left(-\dfrac{1}{3}\right)\times 1\times 1=-\dfrac{1}{3}\]