f(x) = ax² + bx + c
f(1) = a(1)² + b(1) + c = 5
f(1) = a + b + c = 5
f(-1) = a(-1)² + b(-1) + c = 0
a - b + c = 0
So now we have:
f(1) = a + b + c = 5
f(-1) = a - b + c = 0
We add both the equations up,
f(1) + f(-1) :
2a + 2c = 5
a + c = 2.5 // Divide by 2 through
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Answer: a + c = 2.5
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