Melody

Why is arccos (2.3) invalid?

Well  here is a rather pale right angled triangle.

AT is the hyotenuse so it is the longest side

Cos A = AB/AT = little side/longest side

How can you divide a little nummber by a big number and get an answer more than 1?    You can't!!

That is why  cos of an angel must NOT be more than 1  

and that is why arccos (2.3 ) is invalid!

3.14 is an approximation for pi

(66+2/3)/100 of x = 22

(66+2/3)/100 * x = 22

 x = 22 * (100/(66+2/3))

$${\mathtt{22}}{\mathtt{\,\times\,}}\left({\frac{{\mathtt{100}}}{\left({\mathtt{66}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{2}}}{{\mathtt{3}}}}\right)}}\right) = {\mathtt{33}}$$

T2-T1 = T3-T2=t4-T3 =..... = d

Not sure what you mean

$$6\times 10^{-3}=6\times \frac{1}{10^3} = \frac{6}{10^3}$$

Maybe that will help ?

just a little mistake Sasini

280-66=214

$${\mathtt{56}}{\mathtt{\,-\,}}{\mathtt{8}}{\mathtt{\,\times\,}}\left({\frac{{\mathtt{1}}}{{\mathtt{4}}}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{2}}}{{\mathtt{1}}}}{\mathtt{\,-\,}}{\frac{{\mathtt{3}}}{{\mathtt{5}}}}\right) = {\frac{{\mathtt{214}}}{{\mathtt{5}}}} = {\mathtt{42.8}}$$

YOU HAVE TO FLIP THE SECOND ONE UPSIDE DOWN !

$$\frac{3}{7} \div \frac{5}{7} = \frac{3}{7}\times \frac{7}{5} = \frac{3}{5}$$

$$\displaystyle\sum_{n=1}^{5}\;\; \frac{(-1)^{n+1}\;n}{3^n}$$

Maybe this will help a little