Math Program arithmetical progression problem

[zitendr 0]

Short program in c to calculate first term when sum of n terms and common difference d is given.

[seec77 0]

I think you forgot to post the code!

[DrK3055A 0]

I think you forgot to post the code!

This program is inmediate. The sum of an arithmetic serie, when you know the first and last term of a serie of n elements:

S=N*(a[1]+a[N])/2;

but you know that

a[N]=a[1]+(N-1)*d;

Doing arithmetics, then:

a[1]=S/N-(N-1)*d/2;

a[n]=S/N+(N-1)*d/2;

#include <stdio.h>int main(){	double d,s,a1,an;	long n;	printf("\nHow many terms? ");	scanf("%d",&n);	printf("\nSum of all terms? ");	scanf("%lf",&s);	printf("\nCommon difference? ");	scanf("%lf",&d);	a1=s/n-(n-1)*d/2;	an=s/n+(n-1)*d/2;	printf("\nFist term A[1]: %f\nLast term A[%d]: %f\n\nSerie:",a1,n,an);	for(long i=1;i<=n;i++){		printf("%f",a1+(i-1)*d);		if(i<n) printf(", ");		}}

Edited October 21, 2006 by DrK3055A (see edit history)