Cho biết y tỉ lệ thuận với x theo hệ số tỉ lệ k và khi x=4 thì y=12 a) Viết công thức tính y theo x
b) Tính x khi y=9

14

step-by-step explanation:

step-by-step explanation:

the true ones are a b and d

we have three similar triangles, because each has a right angle and shares an angle.   let's write the angles in order: opposite to short leg, long leg, hypotenuse.

cab similar to bad similar to cbd

or as ratios,

ca: ab: cb = ba: ad: bd = cb: bd: cd

we also know

ac = ad + cd

(ad+cd): ab: cb = ba: ad: bd = cb: bd: cd

(ad+cd)/cb=cb/cd

we have cb=6, ad=5 and seek x=cd.

we reject the negative root and conclude x=4

answer: 4