In the parallelogram ABCD, the height lowered to the AB side is 20, AD = 25. Find the sine of angle B.

Let DH be the height dropped to the side AB. Then we get a right-angled triangle ADH, the angle of which AHD is 90 degrees, the leg DH is 20, and the hypotenuse of AD is 25. Then the sine of the angle A will be

sinA = DH / AD = 20/25 = 0.8.

Considering that the sum of the angles A and B is equal to 180 degrees, we get:

sinB = sin (180 – A).

Using the reduction formulas, we get:

sinB = sin (180 – A) = sinA = 0.8.