If you want to calculate the area, any of the three sides can be the base. From the base, you draw a perpendicular line to the third angle, to get the height. The result will be the same (within the limit of drawing accuracy, of course), whatever side you choose as the base.
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You can't calculate any angle if all you know is one side of the triangle.
Use the sine rule to work out one of the sides. (a/sina = b/sinb = c/sinc) Then as it is an isosceles triangle the perpendicular dropped from the apex will (a) bisect the base and (b) form a right angle with the base. Now you know one side and the hypotenuse of a right-angled triangle and you use Pythagoras (a2 + b2 = c2) to solve the 'other' side of that, which is the height of the isosceles triangle.
A triangle with side a: 40, side b: 25, and side c: 25cm has an area of 300cm2
If it is a right angled triangle then it is 0.5*(base)*(height) of the triangle half the base times by the height imagine it as half the area of a rectangle of the two triangles * * * * * That is all very well if you know the base and the height. Sometimes you know only the three sides. Or only two sides and an angle, or two angle and a side. There are a whole host of formulae for such circumstances.
with a tryangle you have 3 sides. A base and a leg and a hypotnuse. if you have a base and an angle just use the trig function to find your hight.