Question

the sum of the AP 40,36,32, 28...…15term

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Answer 1

Step-by-step explanation:

When the A.P,$$40,38,36,34,32....$$ reaches $$0$$ we observe that on adding it does not contribute anything to the sum and the sum even starts decreasing once it reaches $$-2$$ .Therefore,the sum has to be evaluated till $$2$$ .Now,$$\begin{lgathered}a=40\\d=-2\\n=20\\S=\frac{a}{2}[2a+(n-1)d]\\=\frac{20}{2}[2\cdot40+(20-1)(-2)]=10(80-38)=420\end{lgathered}$$