Index Law

**Tzar** · January 3rd 2011, 06:40 AM

I got the answer x^2 + 49 from the question (x+7)^2.
But then i was told the answer was x^2 +14x+49.

could some explain the 14x pleas?

**e^(i*pi)** · January 3rd 2011, 06:45 AM

It [I assume you mean 14x not 40x] because $(a+b)^2 \neq a^2+b^2$

You need to either memorise the result, use the binomial theorem or use the distributive property/FOIL.

Using the distributive property $(x+7)^2 = (x+7)(x+7) = x(x+7) + 7(x+7) = (x^2+7x) + (7x+49) = x^2+14x+49$

Memorising that $(a+b)^2 = a^2+2ab+b^2$ is a great timesaver in exams

**Tzar** · January 3rd 2011, 06:48 AM

Thank you

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