A triangle has sides 35cm;54cm;61cm long.Find area ? Also find the smallest of it's altitude?

It appears that you have not adequately learned the basics of trigonometry. You need to review those principles until they seem obvious. Trigonometry is based on a picture of a circle with the radius making an angle with the x axis. You carry this picture in your head, not your calculator. Trig is mostly imagination and the rest memorization.

Remember Chief SohCahToa
Sin = Opposite/Hypotenuse
Cosine = Adjacent/Hypotenuse
Tangent = Opposite/Adjacent

Also memorize:
Sec = 1/Cos
Csc = 1/Sin
Cot = 1/Tan

Here is the general analysis of the triangle: en.wikipedia.org/wiki/Triangle