Answer:
Hence, the number of ways of doing so is:
780 ways.
Step-by-step explanation:
We know that if we have to choose r items out of a total of 'n' items then the number of ways of doing so is calculated by the formula of combination as:
![n_C_r](https://tex.z-dn.net/?f=n_C_r)
which is given by:
![n_C_r=\dfrac{n!}{r!\times (n-r)!}](https://tex.z-dn.net/?f=n_C_r%3D%5Cdfrac%7Bn%21%7D%7Br%21%5Ctimes%20%28n-r%29%21%7D)
Here we have to chose 2 books out of a shelf of 40 books.
i.e. we have: n=40 and r=2
Hence, the number of ways of doing so is:
![{40}_C_{2}=\dfrac{40!}{2!\times (40-2)!}\\\\\\{40}_C_2=\dfrac{40!}{2!\times 38!}\\\\\\{40}_C_2=\dfrac{40\times 39\times 38!}{2!\times 38!}\\\\\\{40}_C_2=\dfrac{40\times 39}{2}\\\\\\{40}_C_2=780](https://tex.z-dn.net/?f=%7B40%7D_C_%7B2%7D%3D%5Cdfrac%7B40%21%7D%7B2%21%5Ctimes%20%2840-2%29%21%7D%5C%5C%5C%5C%5C%5C%7B40%7D_C_2%3D%5Cdfrac%7B40%21%7D%7B2%21%5Ctimes%2038%21%7D%5C%5C%5C%5C%5C%5C%7B40%7D_C_2%3D%5Cdfrac%7B40%5Ctimes%2039%5Ctimes%2038%21%7D%7B2%21%5Ctimes%2038%21%7D%5C%5C%5C%5C%5C%5C%7B40%7D_C_2%3D%5Cdfrac%7B40%5Ctimes%2039%7D%7B2%7D%5C%5C%5C%5C%5C%5C%7B40%7D_C_2%3D780)
Hence, the answer is:
780
It’s going to be the second one. B
The answer is 94 because the ?angle is the alternating exterior angle of the given angle which makes them congruent.