 Hi and welcome to the session. I am Shashi and I am going to help you with the following question. Question says evaluate sin 18 degrees upon cos 72 degrees. First of all let us understand that if we are given any right triangle and theta is any of the acute angles in that triangle then cos 90 minus theta is equal to sin theta. This is the key idea to solve the given question. Now let us start the solution. We have to find sin 18 degrees upon cos 72 degrees. Now we know 72 degrees and 18 degrees are complementary angles. So we can write sin 18 degrees upon cos 72 degrees is equal to sin 18 degrees upon cos 90 minus 18 degrees. We can write 72 degrees as 90 degrees minus 18 degrees. Now from key idea we know cos 90 minus theta is equal to sin theta. Here value of theta is 18 degrees. So cos 90 minus 18 degrees can be written as sin 18 degrees. So we can write this expression is equal to sin 18 degrees upon sin 18 degrees. Now sin 18 degrees and sin 18 degrees will cancel each other and we get answer 1. Therefore sin 18 degrees upon cos 72 degrees is equal to 1. So our required answer is 1. This completes the session. Hope you understood the solution. Take care and have a nice day.