Does anyone know how to solve number 11? i have the formulas under the question?

but

now substituting it in the formula

now multiplying with the sine alpha throughout the equation

but mathematically

hence proved.

answer: 45

not only in mathematics, scientific proof also are required in the other field of science. also maths proofs are of particular structure, containing steps and logics. so proving is like doing science, it is science. that's why it is important to prove. 

an inaccurate proof leads to errors, or stating results that are not accurate.  which wil mess the probelm up you dont want this