How many ways can you choose 2 books from a shelf of 40 books

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:

which is given by:

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:

            Hence, the answer is:

                   780

the correct option is 2.

step-by-step explanation:

according to the given information triangle abc is a right angle triangle with right angle at a. segment ab is 5 and segment ac is 7. point d is on segment bc and angles adb and adc are right angles.

it is given that triangles abd, cad, and cba are similar.

two triangles are similar if their corresponding sides are proportional.

in triangle cba, using pythagoras theorem

                      )

triangle abd and cba are similar,

                            (2)

triangle cad and cba are similar,

                      )

using (1), (2) and (3), we get

hence proved.

therefore option 2 is correct.