+37153 Draw yourself a picture of this isosceles triangle.....you need to find the height by dropping a line from the apex to the base to form a right triangle.....then you can calculate height:
h^2 + (1/2b)^2 = 25^2
h ^2 = 25^2 - (1/2 (40))^2
h = 15
Now you can calculate sine ACB = opposite/hypotenuse = h/25 = 15/25 = 3/5 = .6
