## anonymous 3 years ago Find S20 when t1 = 4 and d = 5

1. anonymous

Now you have to use formula

2. anonymous

tn=t1+(n-1)d

3. anonymous

$$S_{n}=\frac{n}{2} (2 t_1 + (n-1)d)$$ sub n=20, and everything you've got there. do u need further help?

4. anonymous

cool

5. anonymous

S(20=20/2(2(4)+(20-1)5)

6. anonymous

And how would I work this out @Shadowys

7. anonymous

um,simplify this then. $$\frac{20}{2}[(2)(4)+(20-1)5]=10[8+19(5)]$$ what follows can be done by manual arithmetic or a calculator.

8. anonymous

I got 1030

9. anonymous

yup, that's what I got too. :)

10. anonymous

Cool