# To solve: x + 3.9=4.2

Question
Decimals
To solve: $$\displaystyle{x}\ +\ {3.9}={4.2}$$

2021-02-13
Calculation: $$\displaystyle{x}\ +\ {3.9}={4.2}$$ Substract 3.9 on both sides, $$\displaystyle{x}\ +\ {3.9}\ -\ {3.9}={4.2}\ -\ {3.9}$$
$$\displaystyle{3.9}\ -\ {3.9}$$ Based on the definition, $$\displaystyle{b}{e}{g}\in{\left\lbrace{e}{q}{u}{a}{t}{i}{o}{n}\right\rbrace}{\frac{{{b}{e}{g}\in{\left\lbrace{a}{r}{r}{a}{y}\right\rbrace}{\left[{b}\right]}{\left\lbrace{r}\right\rbrace}+{3.9}\backslash-\ {3.9}{e}{n}{d}{\left\lbrace{a}{r}{r}{a}{y}\right\rbrace}}}{{+{0.0}}}}{e}{n}{d}{\left\lbrace{e}{q}{u}{a}{t}{i}{o}{n}\right\rbrace}$$
$$\displaystyle{3.9}\ -\ {3.9}={0}$$
$$\displaystyle{4.2}\ -\ {3.9}$$ Based on the definition, $$\displaystyle{b}{e}{g}\in{\left\lbrace{e}{q}{u}{a}{t}{i}{o}{n}\right\rbrace}{\frac{{{b}{e}{g}\in{\left\lbrace{a}{r}{r}{a}{y}\right\rbrace}{\left[{b}\right]}{\left\lbrace{r}\right\rbrace}+{4.2}\backslash-\ {3.9}{e}{n}{d}{\left\lbrace{a}{r}{r}{a}{y}\right\rbrace}}}{{+{0.3}}}}{e}{n}{d}{\left\lbrace{e}{q}{u}{a}{t}{i}{o}{n}\right\rbrace}$$
$$\displaystyle{4.2}\ -\ {3.9}={0.3}$$ Substitute $$\displaystyle{3.9}\ -\ {3.9}={0}\ \text{and}\ {4.2}\ -\ {3.9}={0.3}\ \text{in}\ {x}\ +\ {3.9}\ -\ {3.9}={4.2}\ -\ {3.9}$$
$$\displaystyle{x}\ +\ {0}={0.3}$$
$$\displaystyle{x}={0.3}$$ Final solution: $$\displaystyle{x}={0.3}$$

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