answer
.618 18 repeats
Step-by-step explanation:
Answer:
70/5985
Step-by-step explanation:
We know that a quadrilateral needs to have four vertices (or points on the circle). There are always two ways to link the cross — horizontally or vertically. Using my limited knowledge of combinations, we know that choosing four points out of seven equals 35. Multiplying the two ways to connect those lines (again, horizontally and vertically) makes 35*2 = 70 "bow-tie quadrilaterals" that can be formed on the circle using four points. There are 5985 ways four chords can be chosen out of twenty-five chords because C(25,4) equals 5985, so the probability is 70/5985... and then we just need to simplify that fraction.
If you expand out the brackets you get this,
(4+5i)(a+2i) = 4a + (5a)i + 8i - 10
The -10 comes from 5i * 2i.
Squaring i becomes -1.
Let's group the real stuff together,
and imaginary separately,
(4a - 10) + (5a + 8)i
For this to be purely imaginary,
the real part needs to be zero.
Therefore 4a - 10 = 0
Solve for a.
