Master's thorem in time complexity

#1

Maam please explain how to find time complexity using master’s theorem.

#2

for T(n)=7*T(n/2)+an^2
T(n) = aT(n/b) + f(n)
here a = 7, b = 2 , f(n) = an^2
n^(logb a) = n^(log2 7) which is greater than f(n) asymptomatically as 2^2 = 4 and 2^3 = 8

So f(n) is type 1 hence T(n) = theta(n^(log2 7))

#3

Maam why did you write theta(n^log(2 7) instead of Big oh to find time complexity