-1 C, that's the only logical answer I can think of. Or does it have to be in F?
Answer: 259/260 or 0.99615 (depending on which answer format your teacher wants)
There are 10 numbers in the set {0, 1, 2, ..., 9}. There are 26 letters in the set {A, B, C, ..., Z}. Multiply those values: 10*26 = 260. So there are 260 ways to pick a number followed by a letter. One example is 7P.
There is only one way Matthew can get the correct answer, and there are 260 - 1 = 259 ways to get the wrong answer. We divide 259 over 260 to get the probability of getting the incorrect answer, which is 259/260.
If you need this fraction in decimal form, then use a calculator to find that 259/260 = 0.99615 approximately
I equals 70 grams cup because
I'm guessing the series is supposed to be
![\displaystyle\sum_{n=1}^\infty\frac{n^2x^n}{7\cdot14\cdot21\cdot\cdots\cdot(7n)}](https://tex.z-dn.net/?f=%5Cdisplaystyle%5Csum_%7Bn%3D1%7D%5E%5Cinfty%5Cfrac%7Bn%5E2x%5En%7D%7B7%5Ccdot14%5Ccdot21%5Ccdot%5Ccdots%5Ccdot%287n%29%7D)
By the ratio test, the series converges if the following limit is less than 1.
![\displaystyle\lim_{n\to\infty}\left|\frac{\frac{(n+1)^2x^{n+1}}{7\cdot14\cdot21\cdot\cdots\cdot(7n)\cdot(7(n+1))}}{\frac{n^2x^n}{7\cdot14\cdot21\cdot\cdots\cdot(7n)}}\right|](https://tex.z-dn.net/?f=%5Cdisplaystyle%5Clim_%7Bn%5Cto%5Cinfty%7D%5Cleft%7C%5Cfrac%7B%5Cfrac%7B%28n%2B1%29%5E2x%5E%7Bn%2B1%7D%7D%7B7%5Ccdot14%5Ccdot21%5Ccdot%5Ccdots%5Ccdot%287n%29%5Ccdot%287%28n%2B1%29%29%7D%7D%7B%5Cfrac%7Bn%5E2x%5En%7D%7B7%5Ccdot14%5Ccdot21%5Ccdot%5Ccdots%5Ccdot%287n%29%7D%7D%5Cright%7C)
The first
![n](https://tex.z-dn.net/?f=n)
terms in the numerator's denominator cancel with the denominator's denominator:
![\displaystyle\lim_{n\to\infty}\left|\frac{\frac{(n+1)^2x^{n+1}}{7(n+1)}}{n^2x^n}\right|](https://tex.z-dn.net/?f=%5Cdisplaystyle%5Clim_%7Bn%5Cto%5Cinfty%7D%5Cleft%7C%5Cfrac%7B%5Cfrac%7B%28n%2B1%29%5E2x%5E%7Bn%2B1%7D%7D%7B7%28n%2B1%29%7D%7D%7Bn%5E2x%5En%7D%5Cright%7C)
![|x^n|](https://tex.z-dn.net/?f=%7Cx%5En%7C)
also cancels out and the remaining factor of
![|x|](https://tex.z-dn.net/?f=%7Cx%7C)
can be pulled out of the limit (as it doesn't depend on
![n](https://tex.z-dn.net/?f=n)
).
![\displaystyle|x|\lim_{n\to\infty}\left|\frac{\frac{(n+1)^2}{7(n+1)}}{n^2}\right|=|x|\lim_{n\to\infty}\frac{|n+1|}{7n^2}=0](https://tex.z-dn.net/?f=%5Cdisplaystyle%7Cx%7C%5Clim_%7Bn%5Cto%5Cinfty%7D%5Cleft%7C%5Cfrac%7B%5Cfrac%7B%28n%2B1%29%5E2%7D%7B7%28n%2B1%29%7D%7D%7Bn%5E2%7D%5Cright%7C%3D%7Cx%7C%5Clim_%7Bn%5Cto%5Cinfty%7D%5Cfrac%7B%7Cn%2B1%7C%7D%7B7n%5E2%7D%3D0)
which means the series converges everywhere (independently of
![x](https://tex.z-dn.net/?f=x)
), and so the radius of convergence is infinite.