Prove that : `int_(0)^(a) f(x) dx = int_(0)^(a) f(a-x)dx`
hence evaluate : `int_(0)^(pi//2) (sin x)/(sin x + cos x) dx`

Explanation

To prove the statement \( \int_{0}^{a} f(x) \, dx = \int_{0}^{a} f(a-x) \, dx \), we can follow these steps:

### Step 1: Change of Variable
Let \( t = a - x \). Then, we differentiate both sides to find \( dx \):
\[ dx = -dt \]

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