Angle CDE= angle b, so angle DAC= angle DEC, DC=CD.

So the triangle ADC is equal to the triangle ECD, so the angle ADC is equal to 90 degrees.

Because triangle ABC is a right triangle, we can use Pythagorean theorem: the square of AB is equal to the square of AC plus the square of BC, so we can find AB.

According to the projective theorem

Available AC is the middle term in the ratio of AB to AD, so AD equals the square of AC divided by AB equals 1.8.

This is relatively simple. I don't know if you have studied the projective theorem, because it was in high school. There is also a more troublesome proof method, which only uses similar congruences, but it is actually the same as the projective theorem, but it is not very similar. Can you go to /view/735.html? Wtp=tt Look at the projective theorem here, it is easy to learn.

Hehe, the mid-term exam in the second half of the third grade, I didn't expect to forget to have a holiday almost a month later.